A mathematical analysis has been performed on rotor dynamics of a high-speed polygonal mirror scanner motor in digital electrophotography. The rotor is assumed rigid and vertically supported by air bearings with an effective length that is not negligible compared to the rotor length. The model is a four-degree-of-freedom system that includes the gyroscopic effect and nonorthogonal force of the air bearing. The model also includes the effects of longitudinal bearing length and radially unstable magnetic stiffness of a driving motor and/or a magnetic bearing. A simulation program was coded to calculate complex eigenvalues, static and dynamic stability, critical speeds, unbalance responses, and external excitation responses. The results indicated that although the effects of bearing length and magnetic unstable stiffness were ignored in the past, these simplifications result in substantial error for the evaluation of rotor dynamics. The model is utilized to realize high-performance scanner motors.