Recently, magnonic crystals which are the magnetic counterparts of photonic crystals or phononic crystals are growing into a hot area of research. In this paper, a plane-wave expansion method in the field of magnonic crystals is proposed. By using this method, band structures of two-dimensional magnonic crystals composed of ferromagnetic materials Fe and EuO are calculated. The numerical results show that the spin-wave gaps will be generated under a definite filling fraction ratio, and the effect of exchange field on the band structure is larger than that of other field in the effective field.