The input voltage to the direct current series motor is controlled by means of an optimal regulator, for changing the firing angle of the thyristors using pulse width modulation technique. An input output linearization technique is introduced to linearize the non–linear mathematical model of the d.c series motor system .The open-loop and closed-loop control systems hve been analyzed theoretically. A computer program written in a MATLAB is used for computing the system dynamics. The closed-loop control system shows a significant modification in the system parameters like armature current,peak overshoot,settling time……..etc,and an optimal firing angle is achieved.