The purpose of this study are to ensure reliability of the power transmission gears, and to predict operating condition of high speed gears especially with regard to temperature distribution in teeth and blanks. The experiments were carried out using power circulating gear testing machine (1100 kW) under ultra high pitch line velocity (∼150 m/s) and temperature distribution in teeth and blanks were measured. By the experiments, the following facts are known. By increasing line velocity, the temperature of teeth increase parabolically and the distribution of bending stress become nonuniform along face width.