This thesis decribes the design , experiment , development of the speed control of a separately excited DC motor, using a power electronic circuit which controls the power of the motor. The power electronic circuit is the full-controlled rectifier which used a thyristor. In the control of the power electronic circuit , we used the computer (PC) 486 which it ‘ll send and receive any parameter by via Interface Card ,ET-DIO, commanded by Computer. The control program is written by C-language. In result of tests on built system show that the motor worked at average voltage 100 volts on loaded-unloaded condition. The circuit can bared and worked absolutely.