Rigorous coupled wave approach (RCWA) was used to calculate the value of one-dimensional (1D) subwavelenth metal grating′s maximum period that allows only propagation of the zeroth diffraction order (the maximum period is also called critical periodic point). Assuming the surrounding medium is air, some different refractive indexes of the substrate material are chosen to calculate the critical periodic point. It is shown that the critical periodic point is only decided by the refractive indexes of the substrate, no matter which kind of metal, refractive indexes of grating layer, grating period, duty cycle, grating thickness and the polarization and wavelength of incidence are chosen. The results are useful for designing and fabricating novel nano-optics based on 1D subwavelength metal grating by avoiding complicated calculation and the failure of fabricating.