The objectives of this study were to observe morphological changes of root canal walls and to evaluate the capability of Er,Cr:YSGG laser for preparation of curved mandibular incisor root canals in vitro. There are no published reports on the use of Er,Cr:YSGG laser for curved root canal preparation. Twenty curved mandibular incisor root canals (10 and 15 degrees) were prepared sequentially by laser irradiation at 20 Hz, 2 Wusing two different sizes of fine fiber tips (200 and 320 microm in diameters). After laser irradiation, the teeth were bisected longitudinally, and the specimens were observed by stereoscope and scanning electron microscope (SEM). Root canal walls of the irradiated areas showed step-like appearances and openings of dentinal tubules, and root canal preparation, having curvatures of less than 10 degrees, could be carried out by this laser irradiation. Er,Cr:YSGG laser irradiation is capable of preparing root canals having curvatures of less than 10 degrees.