The effect of pivot bearings on the dynamic characteristics of the rotary actuator for head positioning in magnetic disk storage is experimentally investigated. The raiial spring constant and radial damping coefficient of single-row deep-groove ball bearings are obtained by experimental modal analysis. The damping coefficient decreases with increasing bearing axial prelode whereas the stiffness increased, and the damping characteristics strongly depend on bearing lubricating grease. Moreover, a resonance-antiresonance pair due to the pivot bearings appears in the mechanical transfer function of the actuator positioning system. The resonance is well suppressed by the damping effect of the lubricating grease. The results show that the effect of the pivot bearings on the actuator can be accurately predicted from the simple radial stiffness and damping of the bearing.