The essence of in-band transmission loss of plane wave in the Frequency Selective Surface(FSS) with an infinite planar single layer was illustrated by distinguishing the mismatch loss and the dielectric loss.The effects of some parameters on loss characteristics were studied based on a modal analysis approach.Research results show that the dielectric loss is higher when FSS is illuminated with TE polarization in a large incidence angle.While array parameters and aperture widths affect the bandwidth,the dielectric loss always shows the reverse tendency with the bandwidth.The in-band transmission loss of one side dielectric loading is induced mainly by a mismatch loss or by a dielectric loss that depend on the dielectric thickness and the way of dielectric loading.When the dielectric thickness is 1/4 of wavelength,the mismatch loss reaches its maximum;when the dielectric thickness is 1/2 of wavelength,the dielectric loss reaches its maximum.Moreover,with the same dielectric thickness,the mismatch loss of the FSS with dielectric superstratum is less than that of the FSS with a substrate.The loss of both-side symmetric loading is from dielectric loss,which is lower than that of one-side loading.