Abstract: The existing reflection and transmission coefficients of a frequency selective surface(FSS) do not apply to calculation of reflection and transmission under normal incidence of waves,and need to be modified.The incident field was decomposed into two components,parallel and perpendicular to the incidence plane,respectively.Consequently,reflection and transmission coefficients were defined by tangential components of the scatter field.The modified coefficients are simple in structure and applicable to incidence at any angle.A numerical result shows that the modified reflection and transmission coefficients agree with those obtained with the commonly used simulator HFSS.