A FDTD analysis of grid characteristics of millimeter wave quasi-optical array is presented in this paper. The grid impedance and equivalent circuit parameters are obtained by the FDTD simulations through proper treatment of metal grids on the interface between air and GaAs substrate. The comparisons are made among the FDTD results, analytical solutions, and experimental data. provides more accurate results than analytical solutions and proves to be an excellent tool for analyzing quasi-optical device grids.