Real time investigstions on the thermal process of laser-induced damage of a Almirror and a reflectance-increasing Al mirror by photothermal displacement technique (PDT) are described. In these studies, changes of the sisnals of PDT of the damage processof Al mirror and reflectance-increasing Al mirror are observed. The mechanism of thethermal damage of Al mirrors and effect of existing dielectric films on the damage processare discussed.