Abstract: Solid-liquid velocity profile is an important factor to affect the performance of the oil film lubrication. However, the velocity profile distribution of the fluid film lubrication in ambient pressure cannot be determined by effective experimental methods. In the present work, an apparatus has been built for in situ measuring velocity profile of thin oil film under confined micro-gaps by fluorescence photobleaching imaging microscopy technique and imaging the shape evolution of the bleached area under pure shear. The results show that the velocity profile of PB450 was typical linear distribution accordance with the lubrication theory, and there were obvious nonlinear plug velocity profile of poly alpha olefins (PAO6) lubricant film of 8.5 μm in thickness doped with fluorescent agent, probably because of the oil film viscosity decreased near the solid-liquid interface. Under the same sliding velocity, the deviation of velocity profile of the PAO6 oil film from the linear distribution increased along with the decrease of the film thickness. Velocity profile of the lubricant film was in agreement with the experimental results of hydrodynamic lubrication, which enhanced the reliability of the measurement results.