Diffraction grating isindispensable for quantum well infrared photodetectors(QWIP) in order to improve opticalcoupling because of the limitation of quantum selection rules. A numerical approach basedon finite difference time domain(FDTD) method was presented for investigating thediffraction effect of a metal grating fabricated in QWIP. Simulation results demonstratethat such a complicated structure can be optimized by electromagnetic analysis based onthe FDTD method. Detailed field distribution inside QWIP can be obtained for estimatingthe optical coupling efficiency of the grating.