Recently, being due to development of a small microprocessor, microprocessor have found increasing application as a digital controller in the control system. In this paper, authors analyze theoretically the continuous PID controller of a position control system with servomotor, and program the microprocessor as digital PID controller by an assembly language, and search the optimal parameters of the digital PID controller which make the smallest integral square error criterion for a performance criterion, and take experiment the indicial responses with optimal parameter. The results are following. 1) PD- behavior controller was better than P-behavior controller. 2) The smaller the smapling times of P-behavior controller and PD-behavior controller were, the better the indicial responses of the discrete system were. 3) Using a small microprocessor could replace the traditional continuous PID controller for good control.