Abstract: In this paper, an X-band TE01 output window is designed, which is suitable for high power klystron. TE01 window has its uniqueness to avoid voltage breakdown due to its mode pattern and window structure. Center frequency for this window is 9GHz and bandwidth can reach as high as 7% with standing wave ratio below 1.2:1. Theory about heating effect on the designed window has been established, which shows that window cracking due to temperature gap will not occur when high power microwave with its peak value of 50MW, pulse length of 1.5s and repetition rate of 180Hz is passing through.