An after treatment system is disclosed. The after treatment system may include an exhaust pipe through which an exhaust gas flows; a three-way catalyst (TWC) mounted on the exhaust pipe and purifying HC, CO, and NOx contained in the exhaust gas, an ammonia production catalyst (APC) mounted on the exhaust pipe at a downstream of the TWC, storing NOx at a lean air/fuel ratio, and generating H2, releasing the stored NOx, and generating NH3 using the released NOx and the generated H2 at a rich air/fuel ratio, and a selective catalytic reduction (SCR) catalyst mounted on the exhaust pipe at a downstream of the APC, storing the NH3 generated in the TWC and the APC, and reducing the NOx contained in the exhaust gas using the stored NH3.