This paper describes the electromagnetic wave diffraction from a metallic Fourier grating based on the T-matrix analysis and the extended boundary condition. The grating consists of a basic sinusoidal wave and a second-harmonic wave. The analytical form of the expression of matrix elements is presented in the term of the Bessel functions. The error of power conservation versus the truncated number of the basic functions has been examined; thus a high accuracy result is obtained. Diffraction efficiencies versus incident angle, groove depth and wavelength have also been discussed for both perfect and lossy metallic (Au) gratings. Numerical results are in good agreement with those obtained from other methods and experimental values. Reasonable numerical results are presented for a groove depth per period of the grating less than 0.28.