Abstract: S-band tapered magnetically insulated transmission line oscillator designs using slowly tapered vanes and sharply tapered vanes were presented and analyzed. Simulation results show that: this structure can produce microwave at mean power of 2.5 GW and frequency of 2.65 GHz under the diode voltage of 500 kV; the structure improves the efficiency of beam-microwave conversion from 12% of a traditional tapered magnetically insulated transmission line oscillator to 15% . In the following experiments, average power of 1 GW has been obtained at frequency of 2.69 GHz which was in good agreement with the simulation.