To cope with the problem that the thickness and optical constants of thin film can not be measured accurately when deposited on a transparent substrate due to the incoherent reflected-light from the back-side of the substrate, a method is presented to determine the thickness and optical constants of hydrogenated amorphous silicon (a-Si:H) thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) from the transmittance spectra of ellipsometer only. The influence of the substrate temperature (Ts) and the gas temperature (Tg) before glow discharge on the thickness and optical constants of a-Si:H thin films is analysed. The results show that the thickness of a-Si:H thin films determined by transmittance spectra agrees well with that measured by scanning electron microscopy (SEM). The optical constants of the films deduced from the transmittance spectra accord with the results obtained by other researchers. This method can be applied in deposition of amorphous or polycrystal thin films on transparent substrates.