By using the rigorous Fourier modal theory, the influence of the substrate refractive index, the incident angle, the normalized period and the normalized depth with different values on the diffraction efficiency of a sinusoidal grating microstructure was investigated. Also, the diffraction properties of the grating were analyzed. Based on the scalar diffraction theory and the effective medium theory, the diffraction efficiencies of the sinusoidal grating were calcualted at which the period is far greater than the incident wavelength and far smaller than the wavelength respectively. Using the different parameters of grating structure, comparing the results calculated by these two theories with that estimated by Fourier modal theory, the accuracy of the scalar diffraction theory and the effective medium theory was analyzed. The results show that when the refractive index of grating is about 1.5, the normalized depth is less than or equal to 1 and normalized period is greater than or equal to 5, the scalar diffraction theory can be used to calculate the diffraction efficiency of sinusoidal grating, the error is less than 3%; If the refractive index is increased to 3.42, only when the normalized period is greater than or equal to 10, the scalar diffraction theory can be used to estimate the diffraction efficiency and the error is less than 5%. Only zeroth transmittance light is propagating, effective medium theory is valid to evaluate the diffraction efficiency of sinusoidal grating. It is noted that the error of two simple methods is minimum at a normal incidence. Furthermore, the accuracy of scalar diffraction theory and effective medium theory decreases as the incident angle increasing.