The scalar two-dimensional finite difference time domain (FDTD) method is applied to the simulation of light field distribution of waveguide grating couplers. The mode patterns of TE0, TE1, TE2 and TE3 are obtained by using planar optical source as a stimulin. A few important problems are studied carefully, such as: the effect on field distribution with different grating parameters and waveguide structures, the absorbing boundary conditions, etc. The procedure for calculating is compiled in MATLAB. The calculating results are analyzed carefully.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.