By using the effective medium theory, thin film theory, rigorous coupled-wave analysis, and elaborated root finding method, the spectral properties of one-dimensional subwavelength gratings are investigated by using different expressions of the effective refractive index. By using the defined error function of the spectra, the validation of other effective refractive indices is evaluated by using the rigorous refractive index in both cases of TE and TM modes. For the given grating parameters, it is shown that the second order effective refractive index and the exact effective refractive index can provide solid results in estimating the spectral properties of one-dimensional subwavelength gratings for TE mode when the normalized grating period Λ/λ is less than 0.30. For TM mode, the effective refractive indices cannot give solid estimations because more evanescent waves are excited in this case. In addition, it cannot provide solid estimations by using the effective refractive indices in calculating the spectrum as a function of the grating thickness except the second order effective refractive index and the exact effective refractive index for TE mode.