Abstract: With H₂/CH₄/N₂ as the mixed gas source, the single crystal diamonds are deposited on CVD diamond substrate using microwave plasma chemical vapor deposition method. The effect of substrate temperature on diamond crystallization quality and optical properties at a certain concentration of methane are analyzed. Optical microscope and scanning electron microscope (SEM) are used to characterize the color, the internal defect and the surface morphology of the CVD diamond samples. After polishing, the infrared transmittance of the samples is detected by the infrared Fourier transform spectrometer. The results show that the crystallization quality and the infrared transmittance is the best when the growth temperature is maintained at about 930 ℃. The infrared transmittance will reduce if the temperature is higher or lower. It also shows that the diamond grown at a slightly higher temperature has better crystallization quality and infrared transmittance performance than diamond grown at a lower temperature. The N₂ doped diamond shows a brown color and faster growth rate, the infrared transmittance performance of which will be seriously reduced.