The -polarized electromagnetic wave diffraction by semi-infinite periodic venetian blind-type grating is considered. The grating consists of strips placed in the parallel planes. The problem is reduced to a nonlinear operator equation with respect to the unknown reflection operator of a structure. For its solution, a complex approach which consists in using the successive over-relaxation method and the Gauss-Seidel method is suggested. The near and far fields scattered by the grating are calculated. The dependences of the reflection coefficient and diffraction patterns are analyzed for different incidence angles, width of strips and distance between them.Index Terms-Gauss-Seidel method, operator method, regularization procedure, semi-infinite grating (SIG), successive over-relaxation method.