The polynomial identification system is used to estimate the parameter of the DC servo motor. The system is composed the DC servo motor and the electronic circuits. The input signals are composed the series pulse waves. The series pulse waves are different with width and height. The output signal of rotor angular velocity is used to estimate coefficient of the polynomial transfer function. The rotor angular velocities of the open-loop and the close-loop are both verified by the simulation in the study. The real output waveform is approximate to the waveform of the estimated polynomial system. The viscous coefficient and the rotor inertia will be calculated by the transfer function of the polynomial system.