In this paper, we design a novel 2x2 silicon-on-insulator (SOI) waveguide optical switch with X-crossing structure. With free- carrier plasma dispersion effect, we can construct such a device with X-crossing multimode interference region. Applying the X- crossing configuration, the input and output waveguides can be easily laterally separated for direct coupling to fiber without S-bending structure. We use a BPM simulation tool to simulate the output mode patterns and light propagations.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.