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A Relativistic backward wave oscillator (RBWO) is a reliable source of a high power electromagnetic wave (HPEM). The RBWO is capable of high efficiency by a relatively long interaction region between a backward wave and an electron beam. But the pulse shortening problem occurs frequently in experiments of RBWO because of the mode competition, the RF breakdown and so on. After this mode competition between axial modes of TM01, the RF power is vanished. There also occurs a mode competition between TM01 and TM02 when the radii of the hollow electron beam are smaller than a critical value. Mode competition which is one of the main causes of pulse shortening is found to be very sensitive to the position of the electron beam in this RBWO device. Experimental study is done on 100MW RBWO and in progress on X-band hundreds MWRBWO.