Functional recovery for paralyzed patients is realized by repetition of trainings with advice from the therapist. However, in clinical practice, output function of patients is evaluated by qualitative way. For effective training, therapists must quantitatively evaluate the patient's limbs function. To improve this matter, evaluation method of human motion using mechanical power factor is proposed in this study. 'Power factor' has also been used in the electrical system, it can be applied to the mechanical system based on an analogy. In electrical system, power factor represents the percentage of power that is effectively consumed by the load impedance. By using mechanical power factor, impedance characteristics of upper limbs can be acquired. This paper describes the concept of power factor in mechanical system and the evaluation method of upper limb function by using mechanical power factor.