A spatial filter/diplexer for gigawatt level high power microwave is theoretically and experimentally investigated in this paper. The spatial filter consists of several cylindrical rods which are located in the near field of the applied radiating antenna. The S/X dual band microwaves are reflected and transmitted by the spatial filter in the same propagation directions. In the low power experiments, the transmitted and the reflected patterns are very close to that of the applied antenna. The measured reflection and transmission efficiencies, calculated from the power densities in the far-field region, are about 96% and 97% respectively. In addition, the spatial filter has the isolation level higher than 25 dB and the cross polar level much lower than -25 dB. Under the illumination by S/X band high power microwave with a pulse magnitude of about 1.5 GW and duration of about 100 ns, no microwave breakdown is observed.