The robot industry has greatly improved over the past 50 years. It is expected that in the era of the 4th industrial revolution the field of applying robots will expand. Motors are essential in order to operate and control robots. However, robots’ precision requires the application of robot reducers. In particular, many types of harmonic drive reducers have been applied to robots. Harmonic drive reducers have theoretically zero-backlash, but they actually exist for tolerances of Oldham coupling parts. In this paper, dynamic analysis adapted dimensional tolerances of oldham coupling was used to figure out the system backlash characteristics of harmonic drives.