Diamond films with high infrared transmittance have been successfully deposited using CH3OH-H2 gas mixtures through microwave plasma enhanced chemical vapor deposition (MWCVD). The primary purpose of this study is to determine the effect of the deposition conditions on the optical properties of MWCVD diamond films using CH3OH-H2 gas mixtures. Room temperature optical properties of freestanding diamond films were studied by Fourier transform IR spectroscopy. Experimental results indicated that under appropriate deposition temperature (620 ℃) and methanol concentration (5.7%), the refractive index of CVD diamond films (2.33)was comparable with that of natural diamond (2.417). The average infrared transmittance was above 65% in the middle infrared region (500 cm-1 ～ 4000 cm-1), approaching to the theoretical value of diamond (71.4%). The mechanism of growing high IR transmittance diamond films by utilizing CH3OH-H2 gas system is that the high methanol concentration used in this study makes the surface roughness of diamond films decreased by increasing the secondary nucleation density and the high O/C ratio in CH3OH-H2 gas system, improved the quality of diamond films and therefore decreased the absorption of non-diamond carbon in the films.