Properties of the endlessly single mode of photonic crystal fiber(PCF) are analyzed theoretically with the effective-index model. Numerical calculation shows that PCF can be single mode for any wavelength if the proportion of the diameter of the air holes to the pitch in the cladding is small enough. Structure parameters of a single-mode PCF for wavelength above 200 nm are calculated exactly, which provide us with theoretical instruction for the ultroviolet (UV) light single-mode delivery of PCF. Based on 260 nm femtosecond pulses output from a frequency-quadrupled fiber laser, transmission of the UV laser is experimentally demonstrated using a piece of PCF with small air hole. With a core diameter of 4.54 μm, the transmission loss and the coupling efficiency of the PCF are measured to be 0.043 dB/cm and above 31%, respectively.