Abstract: To generate wideband high power microwave with relatively high central frequency, a quarter wave switched oscillator filled with transformer oil as insulating medium is investigated in this paper. Firstly, the structure of this oscillator is introduced. Secondly its characteristic impedance is calculated. Thirdly, the voltage capability is estimated by the simulation results of static electric field distribution and experimental data of the breakdown electric field of the transformer oil. And then the transient performance of the oscillator is simulated by utilizing CST software. The influence of the breakdown position and down-time of the voltage between the switch gap on the peak voltage and central frequency is also investigated. Finally, the experimental result of the oscillator connected to a short helical antenna with a directivity of 3 is introduced. It is shown that the breakdown voltage of the oscillator reaches -322 kV when the rise time of the charging voltage is about 15 ns. The center frequency of the radiated electric field is about 360 MHz with a 3 dB bandwidth of about 22%, while the radiation factor is about 170 kV.