With the rapid development of high power microwave (HPM) technologies, HPM devices with several output frequencies are becoming more and more attractive. Diplexer is a microwave device with two output frequencies, here, an L/X band diplexer with novel structure is tried to be employed in the HPM system. In order to obtain the same radiation direction for the L and X band microwaves in the diplexer, the reflection of L band microwaves and transmission of X band microwaves are realized by an array of irises. To obtain the required performance, the width and thickness of the irises and the distance between them should be well chosen. The diplexer is investigated through theoretical analysis and numerical computation, and the final design described in this work, is the result of an overall optimization process. In simulation, both the reflectivities of L-band microwaves and the transmissivities of X band microwaves reach 99.5%, and the power handling capacity of the diplexer is higher than 5.6 gigawatt (GW). Then, the L/X band diplexer is designed and fabricated according to the optimized results. After that, a series of experiments are carried out to test the diplexer. The cold test results show that the reflectivities of L band microwaves are as high as 97.5%, and the transmissivities of X band microwaves are up to 98.8%, and the acquired radiation patterns show that the designed diplexer is suitable for the diplexing of L/X band microwaves, in a word, the cold test results of the diplexer are in good agreement with the simulational results. In the end, the diplexer is tested with GW class HPMs. The radiated HPMs have little change in the pulse duration and magnitude with the diplexer applied, that is to say, there is no microwave breakdown during the HPM reflection and transmission.