Metamaterials (MMs) are engineered material with electromagnetic properties not commonly seen by natural materials such as negative refraction and near-zero permittivity. These materials have interesting applications ranging from perfect lens and electromagnetic cloaks to leaky-wave antennas and shifters. For such applications, electromagnetic properties of MMs are needed to be known. In this study, we propose a waveguide method for accurate electromagnetic parameter retrieval of weakly-coupled MM slabs (the wavevector is normal to the plane of the metallic inclusions). To achieve our goal, new expressions for effective permittivity and permeability as well as the magneto-electric coupling coefficient are derived. For validation of our waveguide method, we used the simulated scattering parameters of a MM slab composed of split-ring-resonator unit cells by using a commercial 3D electromagnetic simulation program.