A three dimensional printing apparatus including a tank, a modeling platform, a first optical element, a second optical element, and a light source is provided. The modeling platform is movably disposed above the tank so as to move in or out the tank. The first and the second optical elements are respectively disposed next to the tank, wherein the second optical element is located above the first optical element. The light source is located below the tank. After a three dimensional object is formed on the modeling platform, the three dimensional object is moved out of the tank by the modeling platform. A first light generated by the light source is sequentially transmitted by the first and the second optical elements and detour round tank to irradiate the three dimensional object.