In order to realize flat-top transmission property for frequency selective surface (FSS) structure, a novel composite element FSS composed of rectangle metal grid and conventional ring loop element is presented. By applying the spectral-domain approach, the novel free FSS screen is calculated and analyzed theoretically. As a result of the numerical analysis, it is shown that the flat-top passband of 8 GHz and sharper skirts, diplexing closely separated signal bands, for example, X- and Ku-band signals, can be obtained with the novel structures. In addition,the performance of double bands available can maintain stabe for different incidence angles.