Experiments using optical interferometric technique were conducted on a conventional ball-disk test rig to investigate the variation of the oil film in reciprocating motion with zero entraining velocity (ZEV).Driven independently by two individual servomotors,a steel ball and a sapphire disc move at equal speed but in opposite directions in a triangle wave.The oil film thickness between the ball and the sapphire disc was recorded by a camera.After the experiments,the midsection film thickness was measured using a dichromatic interference intensity modulation (DIIM) approach.It is found that the dimpled oil film in transient condition is much shallower than that at steady state with the same load and velocities.Boundary slippage happens in transient reciprocating motion contact under ZEV condition,making the shape and position of the dimple different from the steadystate ZEV dimple.The natural occurrence of the oil starvation produces large area of dry contact and mixed lubricated contact