There are two kinds of heat that can be generated by the meshing of plastic gears. The first is the frictional heat and the second is heat generation by hysteresis. The heat exerts a influence upon the wear of the gears. This heating value varies with the running conditions and the use of the plastic materials. Experiments were carried out to analyze the relation between rotational speed and heating value. It was calculated that the heating value of other plastic materials used for gears, with the ranges of using to the various rotational speeds were decided. As mentioned above, unlike metal gears, the wear of plastic gears is influenced by the heat generated due to hysteresis, because the plastic materials are visco-elastic materials. The results obtained provide some data regarding the design and use of plastic gears.