Based on the rigorous coupled-wave analysis, considering the actual fabrication processes for nanometer level gratings, the first-order diffraction efficiency was simulated for a normally-incident transmission grating at wavelength of 13.4 nm. The influence on the first-order diffraction efficiency by parameters, such as grating materials, relief thickness, gap/period ratio, and trapezoidal angle on the relief are analyzed quantitatively. The results show at the wavelength, the phase of the relief (Si3N4, Cr, Au) effects grating diffraction greatly. The phase shift of the nonmetallic relief (Si3N4) is found to precede the metallic materials (Cr, Au). After optimization, Si3N4, Cr and Au gratings with Si (Si3N4) substrate are proposed , which have the higher first-order diffraction efficiency than the Cr/Si3N4 compound gratings currently used for soft X-ray interference lithography.