Abstract: Input window and output window for 8mm gyroklystron is designed by using three-dimensional electromagnetic code in this paper. The VSWR of rectangular window applied to input window is below 1.08 over the frequency band 34 to 36 GHz, while the VSWR of output window is below 1.2 over the frequency band 34.4 to 35.4 GHz. The effect of dielectric material thickness and relative dielectric constrant on reflection coefficient of RF window is discussed.