Abstract: A compact L-band sealed-tube magnetically insulated transmission line oscillator (MILO) has been developed that does not require bulky external vacuum pump for repetitive operations. A special designed ceramic insulated vacuum interface was designed and the flashover was well controlled. A dynamic 3-D Monte-Carlo (MC) model for the molecular flow movement and collision was setup for the MILO chamber. The pulse desorption, gas evolution and pressure distribution were exactly simulated. In the 5 Hz repetition rate experiments, using a ～30 GW pulse modulator, the average radiated microwave power for 25 shots is about 3 GW in 40 ns pulse duration and the maximum equilibrium pressure is below 4.0×10⁻² Pa. This sealed-tube MILO device is useful for compact and portable high-power microwave applications.