The relationship between the bearing vibration and the sphericity of the steel balls used for ball bearings is experimentally investigated in consideration of the oil film in between the steel balls and raceways. The experiments are carried out on angular contact ball bearings (7208C and 7010C) under only thrust load. First, it is confirmed that the oil film thickness, which is measured on this bearing, virtually agrees with the value calculated by EHL theory. And even if there exists a considerably large error in the ball shape, it does not always necessarily rupture the oil film if it is sufficiently thick. Second, in a simple two-bearing-shaft system, the even-order undulation of form error of ball, especially, second-order undulation, can cause bearing vibration. Furthermore, when the ball has a three-dimensional shape like a rugby ball, with elliptical cross-sections, the ball determines its own rotational axis.