Micro/nano-scale flow phenomena widely exist in both nature and engineering, determining mass, heat and even information transport, and play a major role in the rapid development of novel chip-based techniques and material science, etc. In life phenomena, crucial nutrition and oxygen transport processes rely on the blood flow in capillary vessels with diameters of 5~10 μm. In the engineering field, diverse chips have been developed for bacteria detection, DNA sequencing, pollutant analysis, heating and cooling, medicine synthesis and sensing, etc. In daily life, micro/nanofluidics-based techniques have emerged in wearable devices.All the above applications are inevitably related to the transport process determined by the flow velocity fields, which is the key to understanding the mechanism of flow phenomena and extending their applications. For this reason, in the past decades, various techniques of flow velocity measurement have been developed. In this paper, we briefly introduce some commonly used velocity measurement methods, e.g. micro particle image velocimetry, particle tracking velocimetry, molecular tagging velocimetry, optical coherence tomography, magnetic resonance imaging technique, etc. Most of them are optical methods and compatible with micro/nanofluidics investigations. Subsequently, according to the emerging demand for high temporal and spatial resolution in current micro/nanofluidic investigations, a novel micro/nanofluidics velocity measurement technique-Laser Induced Fluorescence Photobleaching Anemometer (LIFPA) is introduced in detail in this paper.LIFPA is a non-invasive optical technique proposed in 2005. After nearly two decades of development, LIFPA has become a promising micro/nanofluidics velocity measurement method. It has high spatial and temporal resolutions, with ultrahigh sensitivity. With a confocal microscope and a continuous-wave laser, the LIFPA system shows a spatial resolution of ~200 nm. Moreover, with a Stimulated Emission Depletion (STED) microscope, a super-resolution velocity measurement of ~70 nm can be realized. The most recent investigation shows that, if the excitation laser power is sufficiently strong, even if works with a confocal microscope, the velocity measurement by LIFPA can also overcome the diffraction limit and achieve super-resolution. In the meanwhile, the temporal resolution of LIFPA can be up to the order of microseconds. In the practical measurements in microscale flows, 3 kHz velocity fluctuation has been reported. The minimum velocity fluctuation that can be measured is as low as 600 nm/s. All these specifications indicate that LIFPA possesses has the far-field nanoscopic capability to measure both steady and unsteady flow on micro/nanoscales.In this paper, we systematically summarize the principle and theoretical foundation of LIFPA. Then, the major achievements of LIFPA in observing novel micro/nano-scale flow phenomena and revealing their mechanisms are reviewed. For instance, in the investigation of Electrokinetic (EK) flow, for the first time, we demonstrate that strong turbulence can be generated in a microchannel of 130 μm wide. The LIFPA measurements further astonishingly indicate that some macroscale turbulence features, e.g. Kolmogorov -5/3 velocity power spectrum, self-similarity of velocity structure function, intermittency and exponential tails of the probability density function, can also be observed in a microscale EK turbulence. With these new findings, a comprehensive cascade theory of kinetic energy and scalar in EK turbulence can be established in recent years. It is also important to guide the development of high-efficiency active micromixers by EK turbulence. Another example is the investigation of unsteady Electroosmotic Flow (EOF), an old interfacial phenomenon observed by Reus two centuries ago. It is also a typical nanoscale flow phenomenon originating from the Electric Double Layer (EDL) which is normally below 100 nm thick. Since the dimension is too small, previous researches on EOF are primarily through theoretical and numerical analyses. The limited experiments are conducted far from the EDL as well. With LIFPA, we systematically study the instant velocity fluctuations caused by an external electric field. The results indicate that the response of EOF to the external electric field is not so fast as theoretical predictions. The status of the EOF driven by AC electric field can even become chaotic on the EDL if the applied electric field intensity and frequency are sufficiently high. These unprecedented experimental observations indicate our understanding of fluid dynamics on micro/nanoscales is still far from sufficient. Finally, as a developing velocity measurement method, the shortcomings of LIFPA are discussed and some suggestions for its further development have been advanced.It is well known that no technique can be well developed by only a single research group. We hope that the current review on LIFPA can attract the attention of researchers in relevant fields, e.g. optic, fluid mechanics and micro/nanofluidics, to broaden the applications of LIFPA and promote the development of LIFPA.