Perforatability and thermal damages of dental hard tissues with two kinds of YAG laser are investigated for the purpose of making the cavity and forming the abutment tooth. The perforatability of dentin with laser beam depends on the absorptivity for laser beam, the laser power, the pulse duration and the number of laser pulses. Approximately 65% of Er:YAG laser beam is absorbed at irradiated surface of dentin, but the absorptivity for Nd:YAG laser beam is only 17%. High perforatability is accomplished using Er:YAG laser because of its higher absorptivity. There is a linear relation between the perforated depth and the number of pulses. Higher laser power and larger pulse duration improve the perforatability, in the same time, induce thermal damages on dental hard tissue such as thermal cracks and burning of dentin. Lower laser power and smaller pulse duration makes it possible to make fine cavities without thermal cracks and burning in even with Nd:YAG laser.