This paper takes care of window problem in mechanical power factor calcuration. Unlike power factor in electrical system, mechanical power factor contains problem of taking window in the motion. A motion is composed of various wave form which changes over time. On the other hand, wave form of the electrical system is always sinusoidal when it is alternative current system. When thinking about power factor in mechanical system, when to take window over the motion is critical. This paper addresses a problem by calculating power factor on time. Realtime power factor calcuration using short-time Fourier transformation is explained in the paper. Reliability of the calcuration is shown by comparing experimental data with theoretical result.