Mirror made of Al films can yield high reflectance over a broad wavelength range, and have been widely used in spacecraft optical instruments for high quality optical applications. However, such mirrors might be deteriorated under the irradiation of charged particles in the Earth radiation belt. In order to reveal the deterioration mechanism, the change in optical properties of Al film mirrors induced by proton irradiation with less than 200 keV was studied in a vacuum environment with a heat sink Experimental results showed that the proton irradiation led to an obvious degradation of spectral reflectance of the Al film mirror within the wavelength range from 250 to 800 nm. The threshold fluence 1 x 10(16) cm(-2) was found, above which the reflectance decreased greatly with increasing proton fluence when the radiation damage primarily occurred in the Al film. According to the experimental results, a formula for the performance evolution of Al film optical mirrors irradiated with protons is proposed. (C) 2011 Elsevier B.V. All rights reserved.