A novel imaging dynamic light scattering method for nano-particle sizing which greatly reduces the data processing time is proposed. In this method the scattered signal of particles is recorded as images. Two images of spacial distribution of the scattered light signal are taken in a very short time interval, and the correlation coefficient of the two images is calculated, and then the particle size can be determined by the particle size- correlation coefficient curve at the corresponding delay time. Compared to the traditional dynamic light scattering method, the measure time is shortened from hundreds of seconds to microseconds, and the data processing time is shortened to the milliseconds. Three different latex nano-particles (79、482 and 948 nm) are measured in the experiments of nano-particles, and the measurement error is less than 7%, which illustrates that the real-time online measurement and data-processing can be truly realized.