In this research we investigated some characteristics of rolling friction in the grease- or oil-lubricated ball bearings by using several kinds of single-acting thrust ball bearing. The results obtained are as follows : (1) Friction in thrust ball bearing is nearly equal to the sum of rolling friction between balls and races and sliding friction between balls and retainer. (2) The ratio of the latter to the total friction is approximately less than 20%, and this percentage varies a little with lubricants. (3) Rolling friction force between ball and race is proportional to the area of their contact surface in some ordinary range of load, but in other ranges greater than in this relation. (4) These characteristics of rolling friction are also effected by the kind of lubricants, and the so-called oiliness is much important. (5) From the above-mentioned view point, we can understand the well-known tendencies on frictional characteristics of thrust ball bearings.