Abstract: The highfrequency characteristics of highpower millimeterwave diffraction radiation generator are analyzed in detail using both linear theory and PIC method. This paper presents a method of mode selection. Results show that highpower millimeter wave with good frequency spectrum purity can be obtained by reasonably adjusting the operating parameters such as the electron beam's current, voltage and the highfrequency structure's depth, radius, period. Through analyses it can be found that the device with h=0.05cm can generate singlefrequency and highpower radiation of 8mm wave band. Studies indicate that diffraction radiation generator with simpler structure can also generate singlefrequency and highpower radiation of millimeterwave band.