The essence of in-band transmission loss of plane wave in infinite planar single layer frequency selective surfaces(FSSs) was illustrated with the distinctions of mismatch transmission loss and dielectric loss. The effects of some parameters on loss characteristics were studied based on modal analysis approach. The loss is high when FSS is illuminated with TE polarization and large incidence angle. The array parameters and aperture width affect the dielectric loss

which always runs in the reverse tendency with bandwidth. Whether the in-band transmission loss of one side dielectric loading is induced mainly by dielectric loss or by in-band reflection depends on the dielectric thickness

but the in-band reflection of FSS with dielectric superstratum is less than substrate. The loss of both sides symmetric loading is due to dielectric loss

which is lower than that of one side loading.