The 340 GHz window for Back-ward Wave Oscillator(BWO) is studied in the paper. The simulation results show that the reflect coefficient(S11) is better than 30 dB from 327 GHz-347 GHz. The window is metalized and airtight. The test results of the window show that the S11 is better than 10 dB from 333 GHz-367 GHz, and better than 15 dB from 336 GHz-344 GHz; the loss of the transmission is about 7 dB. The difference between simulation and test is analyzed, which is helpful for the improvement of the window.