Abstract: This paper presents the features, including the radiation pattern and power, as well as the mode information, of the high power microwaves radiated by a reflex triode operating in a laboratory, which are obtained through an improved farfield measurement method put forward here. The measured results show that the radiation power exceeds 300MW with a frequency of 2.8GHz and a pulse width of about 150ns, and that several modes are generated with TE11 mode included.