Abstract: A novel diplexer for the coupling output of GW level microwaves has been investigated, which consists of an array of periodically spaced cylindrical rods. The factors influencing the selection of spatial harmonics for the diplexer were researched, and some important rules for the design of the diplexer were obtained. A cylindricalrod diplexer for the coupling output of S/X band GW level microwaves was designed accordingly. The low power experiments revealed that, its S-band reflection and X-band transmission efficiencies were both higher than 97%. In further high power experiments, the diplexer was operated with microwaves of 1.8 GW and 80 ns, and no microwave breakdown was observed.