Self_modulation may appear on an intense relativistic electron beam(IREB) trave rsing a two_cavity klystron system. It is caused by the intense relativistic eff ect. RF output can be obtained without RF input. Based on this mechanism, a new type of high_power microwave source is suggested. The oscillation condition of t he intense relativistic election beam, traverse mechanism and PIC simulation are presented. After optimization design, a 800MW output peak microwave power was o bserved at a frequency of 099GHz, while an electron beam of voltage 500kV, cur rent 4kA is injected. The efficiency is about 40%.