In this paper, we present a formulation based on surface impedance concept to model reactive impedance surface (RIS) applying the MoM-GEC method (MoM combined to generalized equivalent circuit). The study of the whole periodic infinite RIS is reduced to the unit cell modeling. We are interested especially in demonstrate the RIS impedance behavior against incidence and both TE and TM polarizations. Also, the effect of substrate dielectric permittivity is achieved. Obtained results are given and discussed.