The transmission upper limit of a double-layer frequency selective surface (FSS) with two infinitely thin metal arrays is pre-sented based on the study of the general equivalent transmission line model of a double-layer FSS. Results of theoretical analyses, numerical simulations and experiments show that this transmis-sion upper limit is independent of the array and the element, which indicates that it is impossible to achieve a transmission upper limit higher than this one under a given incident and dielectric-supporting condition by the design of the periodic array. Both the applicable condition and the possible application of the transmis-sion upper limit are discussed. The results show that the transmis-sion upper limit not only has a good reachability, but also provides a key to effectively improve the transmission performance of a double-layer FSS or more complex frequency selective structures.