Abstract: A type of improved C-band MILO is brought forward and its optimized model is designed through theoretic analysis and numerical simulations. According to theoretical analysis of the slow-wave structure (SWS) using the magnetically insulation principle, the cathode radius and primary SWS parameters of C-band MILO are chosen. Through numerical simulations by a 2.5-D PIC code, the impacts of several structure parameters of MILO on its output power are investigated. With 21GW(at 500kV) input-power, a 3.91GHz, 2.77GW average output power microwave is generated from the optimized MILO by simulation. The saturation time is 10ns. The average efficiency is over 13%．