Differential gears play an importat part in many fields and are widely used. Therefore, it is very important to know the dynamic behavior of differential gears, In the present paper, the dynamic behavior of differential gears were investigated by measuring the torque variations of one drive and two driven shafts under conditions where one driven shaft was rotated at a fixed speed, and the other at a variable speed by varying the speed of the drive shaft. Furtheremore, under conditions where hypoid gear was assembled with complete tooth bearing, heel tooth bearing, or hard heel tooth bearing, the dynamic behavior of differential gears was investigated by measuring the tooth root stresses of hypoid gear was and the torque variations of each shaft. In comparison with these measured data, the effect of tooth bearing on the dynamic behavior of differential gears became clear.