In order to measure dynamic loads on gear teeth, a strain gauge apparatus and a dynamical gear testing machine have been made. The strains at the tooth flank, where a strain gauge is attached, are observed and dynamic loads are measured by comparing dynamic strains with static strains. Dynamic behaviors of mating gears are analysed as those of a vibration system which is constructed by the masses of the pinion and gear and the combined stiffness of the mating teeth. Here, the variations of combined stiffness of mating teeth, tooth errors and contact ratio etc. are analysed. The results of the analyses show that measured dynamic loads or measured dynamic stress cycles nearly coincide with calculated results respectively.