Thermal effects on human dentin crystals after exposed to a Nd:YAG laser irradiation have been investigated. Dentin discs specimens from extracted sound human third molar were treated with a Nd:YAG laser pulse at 1060 nm wavelength. The laser was scanned across the specimen with a 200 mum diameter fiber optic delivery probe in contact mode. The irradiance energy used were 40, 60, 80, 100, 120, 140, and 160 mJ/pulse with a repetition rate of 10Hz (pps) respectively. X-ray diffraction measurement was performed on unirradiated and Nd:YAG-treated surfaces. The crystallinity of dentin materials were calculated after profile fitting using a standard quartz sample. Scanning electron microscope equipped with an Energy-dispersive analyzer were used respectively to study dentin surfaces before and after laser treatment. Results showed that crystallinity of hydroxyapatite in dentin increased due to high laser energy outputs. Moreover, intensity of diffraction peaks for dentin hydroxyapatites showed an intensive increase associated with a progressive increase in crystallite sizes which thus representing crystal gr0wth upon laser treatment. SEM analysis documented unique surface morphology for all treatment conditions. It is concluded that laser treatment is to alter dentin materials physically and chemically and this further affects to the microstructure and properties relationship as a substrate for bonding restorative materials.