The air-oil two-phase flow inside an oil-jet lubricated ball bearing was investigated by the CFD method. The VOF method for multiphase flow was used to track the air-oil two-phase flow. The sliding mesh plane was applied between the flow field inside the bearing and the flow field on both outsides of the bearing. The results suggest that the air-oil distribution inside the oil-jet lubricated ball bearing is not uniform. The lowest oil volume fraction appears in the upstream side near the nozzle. The flow velocity and the pressure of the air-oil phase increase inside a faster bearing. The average oil volume fraction becomes lower as the speed increases. With the increase of the oil flow rate, the oil volume fraction in the bearing and the drag against the rotation of the rolling elements and cage increase. The nonuniformity of the air-oil distribution is also enhanced. The nonuniform air-oil distribution should be considered for the heat transfer analysis of the oil-jet lubricated ball bearing. Optimizing the oil-jet lubrication according to the air-oil distribution inside the oil-jet lubricated ball bearing is significant for the high-speed rotation to achieve lower drag loss and greater cooling effect. (C) 2013 Elsevier Ltd. All rights reserved.