Abstract: Experimental studies on the transmittance and reflectivity of COIL mirrors have been done using of the HITACHI U-4001 spectrophotometer and the high reflectivity meter based on cavity ring down spectroscopy, such as experiments of the effects of film uniformity, pollution, and so on. Experimental results show that the ion beam sputtered film is more stable than the electron gun evaporated film. The deposition time has no obvious effect on transmittance and reflectivity of films when the mirrors are put in the dry place.