A novel magnetically insulated transmission line oscillator(MILO) in which a modified HEM 11 mode is taken as its main interaction mode(HEM 11 mode MILO) is simulated and experimented in this paper.The excitation of the oscillation mode is made possible by carefully adjusting the arrangement of each resonant cavity in a two-dimensional slow wave structure.The special feature of such a device is that in the slow-wave-structure region,the interaction mode is HEM 11 mode which is a TM-like one that could interact with electron beams effectively;and in the coaxial output region,the microwave mode is TE 11 mode which has a favourable field density pattern to be directly radiated.Employing an electron beam of about 441 kV and 39.7 kA,the HEM 11 mode MILO generates a high power microwave output of about 1.47 GW at 1.45 GHz in particle-in-cell simulation.The power conversion efficiency is about 8.4 % and the generated microwave is in a TE 11-like circular polarization mode.In a preliminary experiment investigation,high power microwave is detected from the device with a frequency of 1.46 GHz,an output energy of 43 J-47 J,and a pulse duration of 44 ns-49 ns when the input voltage is 430 kV-450 kV,and the diode current is 37 kA-39 kA. A novel magnetically insulated transmission line oscillator (MILO) in which a modified HEM 11 mode is taken as its main interaction mode (HEM 11 mode MILO) is simulated and experimented in this paper. The excitation of the oscillation mode is made by. the arrangement of each resonant cavity in a two-dimensional slow wave structure. The special feature of such a device is that in the slow-wave-structure region, the interaction mode is HEM 11 mode which is a TM-like one that could or interact with electron beams effectively; and in the coaxial output region, the microwave mode is TE 11 mode which has a favourable field density pattern to be directly radiated. Employing an electron beam of about 441 kV and 39.7 kA, the HEM 11 mode MILO generated a high power microwave output of about 1.47 GW at 1.45 GHz in particle-in-cell simulation. The power conversion efficiency is about 8.4% and the generated microwave is in a TE 11-like circular polarization mode. In a preliminary exp eriment investigation, high power microwave is detected from the device with a frequency of 1.46 GHz, an output energy of 43 J-47 J, and a pulse duration of 44 ns-49 ns when the input voltage is 430 kV-450 kV, and the diode current is 37 kA-39 kA.