A fuel cell system is provided which produces improved reaction speed and performance. The fuel cell system includes a fuel cell stack including an anode, a cathode, and an electrolyte membrane disposed therebetween. A fuel tank supplies hydrogen-based fuel to the anode of the fuel cell stack, and an oxidant supplying unit adds ozone to oxygen- including air supplies it to the cathode of the fuel cell stack. The ozone supplied to the cathode of the fuel cell stack accelerates a reaction speed in the fuel cell stack, thereby producing a relatively high current density and improving fuel cell performance.