For several decades, intensive research of tooth surface strength of gears has been undertaken to achieve better gear design. Because the practical usage of gears with high hardness has recently increased considerably, a more fundamental research on tooth surface strength of gears with high hardness is required to increase the load-carrying capacity of gears. In view of this present state of research, the authors carried out many experiments in depth mainly on the following topics: (1) tooth surface strength using various lubricating oils in various conditions, (2) tooth surface strength under various effective depths of case-hardening of tooth surfaces, (3) tooth surface strength using test gears with shot peening treatment, and (4) tooth surface strength under various tooth surface treatments (through hardening and nitriding), and (5) tooth surface strength as a function of tooth hardness.