A compact TE/TM polarization beam splitter (PBS) based on a silicon-on-insulator (SOI) ridge nanowire directional coupler is designed and optimized by using a finite-element method (FEM) and a finite difference time domain (FDTD) method. Considering the fabrication precision and the mode mismatching loss in a directional coupler,a gap width about 100 nm is chosen. The ridge height,the ridge width and the gap of two parallel nanowires are optimized to have the shortest length for the polarization splitter. The numerical simulations show that the optimized PBS has a short length of about 17.3 μm,and the waveguide width has a fabrication tolerance of about－20-10 nm,and the bandwidth is about 50 nm when the extinction ratios for both polarizations are larger than 15 dB.