We propose a kind of magnetic photonic crystal (MPC) slab waveguide made from magneto-optical material, namely bismuth iron garnet. The finite difference beam propagation method is employed for numerical calculations, which is the most widely used propagation technique for modeling integrated and photonic devices. The mode conversion and the influence of corresponding parameters are studied theoretically. As a numerical example, a TE-TM mode conversion waveguide-type optical isolator based on this designed MPC is described. An effective away to enhance the Faraday rotation at the 1.55-μm telecommunication wavelength is shown.