Heating source of Our company’s FCEV was only high voltage PTC heater. This system has the merit of quick heating compared with existing heater system using waste heat energy of vehicle’s engine. But the system has the cold RAM AIR inflow problem to the passenger’s side at blower OFF and the fuel efficency falling-off problem (40.2%↓) by using 5㎾ high voltage PTC for heater operation. This study developed our company’s first hot-water heater using stack coolant in the second stage of MOHAVE FCEV and improved heating performance and fuel efficiency (max 38%↓) by combination of hot-water heater and PTC heater at optimal condition, and proposed guide about the development direction of FCEV, the next generation’s environment-friendly vehicle’s heater system.