To overcome slow leaking problems of the Micro-Crystalline Diamond(MCD) window, a study on an extremely thin composite diamonds for the windows of the Terahertz(THz) wave Traveling Wave Tubes(TWTs) is presented. The composite diamond is prepared by Microwave Plasma Chemical Vapor Deposition(MPCVD) method. To reinforce the mechanical properties and air tightness, MCD is deposited incorporated with Ultra-Nano-Crystalline Diamond(UNCD). A 100 μm thick composite film is designed and fabricated. The fracture strengths of the composite films with thin layers of UNCD are 2-3 times higher than those of the MCD films. The results of the leak test show the maximum leak rate of the windows is less than 1×10-10 Pa?m3/s. 180 GHz and 220 GHz TWT composite diamonds windows are assembled. Results of the Radio Frequency(RF) tests of the two windows show that S11≤-15 dB(in range of 10 GHz) and S11≤ -10 dB(in range of 20 GHz) respectively, indicating a good RF performance. This work provides a novel approach for the fabrication of extremely thin diamond windows for THz TWTs.