An improvement in the performance of gears in the drive train is required due to the recent shift to electric vehicles. In this study, directed energy deposition (DED), a type of additive manufacturing, was utilized as a coating technology to get higher-performance gears. However, the mechanical properties of coating layers and heat-affected zone (HAZ) depend on the thermal histories. Therefore, a liquid cooling system consisting of a heatsink and a chiller which can adjust water temperature and flow rate was developed to improve the hardness of gear teeth by coating with DED. The performance of the liquid cooling system was evaluated by the temperature transition during the coating process and by measuring the hardness of the coating layer. The combination of the developed liquid cooling system and thermal conduction grease reduced the maximum temperature and exhibited a higher cooling rate during the process. As a result, the hardness of the gear teeth was successfully increased.