Electromagnetic properties of a two-dimensional metallic photonic crystal in a transmission line is analyzed, and a compact TEM-TE11 high-power microwave mode converter which takes this type of sturcture for phase-shiftting is presented. An L band TEM-TE11 mode converter is optimized using the commercial software cst microwave studio. Its conversion efficiency is 98% at the center frequency of 1.58 GHz. Over the frequency range of 1.56–1.625 GHz, the conversion efficiency exceeds 90%, with a corresponding bandwidth of 4.1%. This mode converter has a gagawatt level power handling capability, thus it is suitable for narrow band high-power microwave application. Using magnetically insulated transmission line oscillator as a high-power microwave source, particle-in-cell simulation is carried out to test the performance of the mode converter. It is found that the proposed mode converter works well with this narrow band high power microwave device.