This paper focuses on improvement of air heater gear design at Ramin power plant, located in southwest of Iran, which failed periodically. Based on hardness measurement results, the mechanical design of gear teeth was studied using AGMA and BS standards. The results showed predicted contact fatigue would occur gradually on the gear teeth. Since the air heater is of a sprocket system, DIN 8196 was also utilized for checking tooth flank profile. A considerable difference between the existing profile and the one produced by DIN instruction was seen. Increasing the surface hardness of tooth was experimentally investigated according to heat treatment procedures. Through investigating the effect of flank profile and hardness on durability of the tooth, it was seen that considering high values for both surface hardness and case depth and using new profile, will improve the endurance of the teeth.