A calculating model of ball pocket rotation and mechanics analyses is established to investigate hydrodynamic lubrication between the ball and the straight sided cage pocket in a ball bearing. An algorithm for calculating ball pocket oil film pressure and film thickness is developed both for a bearing's starting stage and steady running stage. Effects of inner ring rotational speed, cage angular acceleration, and cage's geometry on ball pocket oil film performance are numerically studied by choosing a SKF61928MA bearing as the calculating example. Influences of cage's geometry on ball pocket oil film performance are also investigated. The results show that ball pocket minimum oil film remains unchanged during bearing's steady running stage, while in bearing's starting stage, the larger the cage's acceleration, the smaller the minimum oil film will be.