This paper considered the energy saving problem of D.C. motor in arbitrary load condition. We can improve efficiency by maintaining the proper ratio of armature current/field current. First, the condition of maximizing the efficiency was obtained, and then the real time control was implemented by using microprocessors. As a result, the efficiency was improved in spite of constant output. For example, the efficiency at 1/5 load was improved from 56.7% to 80%.