In designing of high power satellite communication antenna, the high precise design is necessary to avoid multipactor generation under the condition of high vacuum space environment. Especially, realizing high power-handling diplexer, which separates transmitting and receiving power, os required for antenna feed circuit. This paper proposes a new configuration of high power handling diplexer for satellite onboard application and test results in Ku-band. For surviving high power handling, two methods are applied. One is input power to band pass filter (BPF) is divided into half power using hybrid coupler, and the other is input power of stub of BPF is divided into half power using double sided stub configuration. This configuration can decrease to a quarter power of input to avoid power concentration. And design method of stub filter with optimized procedure to obtain good isolation performance in receiving band is described. Drastic improvement can be achieved by introducing this technique. Further, electric field is also calculated using simulator and power limitation to multipactor is estimated. A diplexer has been designed and manufactured in the Ku-band and its design effectiveness has been confirmed through test results.