[en] In the peniotron oscillator, the kinetic energy of relativistic electrons can be efficiently converted to an electromagnetic wave. This feature makes the peniotron a very attractive source of high power microwaves, which is required in heating of fusion plasma in tokamaks and high gradient particle accelerators. In this report we present the design of a 2.75 GHz relativistic peniotron oscillator which is capable of generating a microwave radiation of 30 megawatt with an efficiency of about 60 %. The experimental test results of the designed cavity are also presented. (author)