Abstract: Rolling bearing is one of the most important rotary components in mechanical equipment. It has been commonly accepted that the lubricating performance is intrinsically linked to the stable and long-life operation of rolling bearings, while the distribution and variation of lubricating oil in bearings have a significant influence on the lubrication performance of rolling bearings. A ball-on-disc test rig has been widely used to study the lubrication performance and mechanism as it is easy to control the various operating conditions manually. In contrast, the lubrication in a rolling bearing is less understood. In practice, the lubrication inside a rolling bearing is quite different from that of a single ball-on-disc model because the rolling bearing contains multiple parts and works at more complex conditions. Therefore, it is difficult to directly apply single-point lubrication theories to rolling bearings. 　In this paper, a custom-made rolling bearing test rig was developed to simulate the multiple contacts of real rolling bearings and to study the lubrication performance. A commercial deep groove ball bearing was used as the test bearing. The steel outer ring of the test bearing was replaced by a glass ring to provide an optical window. The number of balls in the test bearing was 12. The test bearing was radially loaded. The inner ring was driven by a servo motor. The oil was tagged with a fluorescent dye, which emitted fluorescent light induced by laser, and therefore the laser induced fluorescence method can be used to observe and measure the distribution of lubricating oil near the ball-outer ring contact region in the rolling bearing. The film thickness of the oil was proportional to the fluorescent intensity under certain conditions. The relation between the film thickness of the oil and the fluorescent intensity received by the high speed camera was determined based on static film thickness calibration. As a result, three-dimensional distribution map of oil in the rolling bearing was obtained at speed range of 6 r/min to 1000 r/min. 　Based on the fluorescence technique, the oil distributed on the balls, the cage and in the pocket can all be found. This paper mainly discussed the influence of different oil supply volume and rotating speed on the distribution of oil reservoir near the contact formed by the outer ring and the rolling balls in the bearing. The results showed that the oil reservoir for each contact was independent oval oil pool at lower speeds while the oil reservoirs of adjacent contacts can be connected with each other through an oil string at higher speeds. The width of the oil reservoir as well as the oil string got wider with the increase of rotating speed of the inner ring under fully flooded lubrication conditions. A cavitation was found to split the outlet zone of oil reservoir. As the adjacent contact was linked with oil string, the cavitation gradually elongated with the increasing rotating speed and got close to the inlet zone of the next contact. The apparent oil reservoir increased with the increase of oil supply; however, the effective inlet oil supply layer may not increase. The thickness of the inlet oil supply layer increased firstly with rotating speed and started to decrease at high speeds. It was found that the extension of the cavitation from previous contact to next contact zone was the main cause of the reduction of the inlet layer thickness. The study of oil distribution presented in this paper can shed some light on certain aspects of the oil supply in oil-lubricated bearings, thereby contributing to the further development of the lubrication theory for rolling bearings.