Two-photon excitation fluorescence (TPEF) and second-harmonic generation (SHG) are detected through multiphoton microscopy (MPM). The major signals have the potential to monitor the process of tissue changes. TPEF and SHG are used to monitor the skin photo-thermal response to irradiation with intense pulsed light sources (\lambda is in the range of 560?1200 nm) and trace the process of skin remodeling in vivo at different time intervals. TPEF intensity is nearly unchanged at different time intervals after irradiation, whereas SHG intensity changes considerably. The results reveal the photo-thermal effect of nonablative light sources and the process of collagen remodeling at the sub-micron level.