We present the design and simulation of an ultra-compact high-efficiency polarization beam splitter (PBS) based on self-collimation effect and photonic crystal band gap in two dimensional photonic crystals. The splitting properties of the PBS have been numerically studied using the finite difference time domain (FDTD) method. It was shown that a 90° separating angle and efficiency of greater than 85% for TE- and TM- modes over a wide frequency range 0.268—0.278(c/a) can be obtained. The size of PBS is only 9 μm×9 μm at optical communication wavelength λ＝1.55 μm. These features of the proposed PBS make it a promising candidate in optical communications application.