The fly-back converter is used to obtain a constant output power switching converter,this paper introduce an approach for speed control of a series DC motor using an AC/ DC converter. This approach is based on the cascade combination of a diode bridge rectifier and fly-back converter. The converter can be operates as buck or buck-boost converter according to the state variable of controlled switch operation. The supply current is approximately sinusoidal with a high power factor. The output voltage and current is achieved with a ripple free. Modeling and simulation of the system are performed using the differential equations which describing the system behavior in different conditions. The motor performance characteristic in different state of operation is controlled by controlling the duty ratio of the MOSFET. The simulation results are a good agreement when compared with experimental results.