Abstract: In this paper, the operation of a relativistic orotron is briefly described. Some preliminary experimental results of a 8 -mm relativistic orotron is reported. Cold and hot measurements of operating wavelength for this device are in good agreement with theoretical calculations. The measured power of E₁₃₁ mode is of the order of 10kW. The relations between output power and guiding magnetic field or pulse voltage are consistent with the expected ones. The improvemen of output power is discussed as well.