Abstract: Based on the bifrequency magnetically insulated transmission line oscillator (BFMILO), a multi-frequency magnetically insulated transmission line oscillator (MFMILO) was put forward. An MFMILO was designed based on the single-frequency MILO, and its high frequencystructure was optimized through electromagnetic simulation. According to different structure parameters, an MFMILO can generate high power microwaves with one-frequency, two-frequency, three-frequency, four-frequency and five-frequency. Simulation results show that an MFMILO can generate microwaves with multi-frequency, and their power efficiencies are less than that of single-frequency MILO. The reasons of less power efficiency are analyzed. The possibility to enhance the power efficiency and frequency quantity of output microw