The optical field distribution of TEo in the waveguide grating coupler is obtained by the vector two-dimensional finite difference time domain (FDTD) method. The calculated results indicate that the same optical field distribution and stable mode pattern can be obtainable through the different kinds of driving sources. It is found that the optical mode is determined by waveguide structure and dielectric parameters rather than the categories of incident light sources. These results are direct-viewing and accurate, and can be quickly obtained. According to the mode field distribution, the optimum coupling efficiency can be predicted. The results by this method agree very well with those of published papers by different algorithms, but the CPU-time and memory elements of computer used by FDTD are much less.© (2007) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.