Abstract: By geometric and mechanics analysis on the angular contact ball bearing, the calculation models of single ball deformation, bearing deformation, load zone and axial preload displacement are established, and the model for calculating the minimum preload for ensuring all balls contact with raceways is established with considering size errors of balls and raceways. The models for calculating the stiffness and damping of bearing are developed. Taking the Jeffcott rotor supported by the angular ball bearing 7012C as research example and by numerical simulation, the influences of preload and size error on the rotor vibration properties are investigated in detail. The vibration spectrum diagram and a set of curves about the maximum amplitude and critical speed related to the axial preload displacement are obtained and analyzed with and without considering size errors of roller and raceway.