Abstract: A compact Lband magnetically insulated transmission line oscillator (MILO) is configured with the reduction of the radius and the length of the cathode, the anodecathode gap, the length and the thickness of the vanes and the period of the slowwave structure. With the use of a 2.5 dimensional fully electromagnetic particleincell code, high power microwave is generated from the device with the average output power of 5.５GW, the frequency of 1.9GHz and the efficiency of 12%. Breakdown can be avoided in the compact MILO and the symmetry of beam emission can be controlled easily.