This paper presents a numerical method to analyze the electromechanical-coupled field in the spindle motor of a computer hard drive and investigates dynamic response due to the electromechanical excitation, i.e. unbalanced magnetic force and centrifugal force for the rotational asymmetric motor. Magnetic field is calculated from Maxwells equation and voltage equation by introducing nonlinear time-dependent finite element analysis. Mechanical motion of rotor is calculated by solving Newton-Euler equation. Electromechanical excitation and dynamic response are characterized by analyzing the free response of a rotating rotor and Fourier analysis of the excitation force and resulting vibration of a rotor. It shows that centrifugal force produces the unbalanced magnetic force even in the rotational symmetric motor. It also shows that resonance produces quite considerable vibration even when the high excitation frequency with small amplitude matches with the natural frequency of the spindle motor.