Plastic gears are more and more widely used in many industrial machine elements. Plastic gear has higher properties such as light weight, wear resistance, and vibration absorbing ability than metallic gears. But, in case of using an inaccurate plastic gear, its tooth breakage happen and fatigue life is shortened due to increase of applying load and temperature rising on the tooth flank. Inaccuracy of plastic gears such as pitch circle roundness and tooth profile generates vibration and noise. In this study, an internal plastic gears which is molded by a new injection mold structure are developed. The new mold structure is called the HR3P(hot runner type 3plate mold) that has an improved runner system in order to have good filling balance. As a result from this study, an internal gear with very accurate roundness was developed by using design of experiment.