A Ku-band magnetically insulated transmission line oscillator is investigated numerically and experimentally for the first time. The magnetically insulated transmission line oscillator (MILO) is optimized in particle-in-cell simulation software. Reflect cavities are introduced to reduce the field intensity between cathode and anode. In the particle-in-cell simulation, the Ku-band MILO generates microwave with power of 2.48 GW and frequency of 12.5 GHz, whereas the voltage is 478 kV and the current is 48.9 kA. Based on the structure in simulation, an experiment system is designed and the measure method and system are introduced. After the collector improved, Ku-band microwave with frequency of 12.9 GHz is generated, whereas the voltage is 539 kV and current is 57 kA. The peak power is 89 MW and pulse width is 15 ns. The radiation mode is TM01 mode.