Abstract Recently, the applications of CO 2 laser irradiation have been investigated for endodontic therapy. The purpose of this study was to evaluate the temperature elevation on the root surface when CO 2 laser was irradiated in the root canal. Ten extracted human teeth with a single root were used. The CO 2 laser tip was placed at the orifice of the root canal and the laser was irradiated in the root canal under two intracanal conditions (in water and without water). The temperature elevation on the root surface was measured at the different parts of the root apex and the lateral surface of the orifice of the root canal by infrared thermometer. The temperature elevation at the root apex was higher than that at the lateral surface of the orifice of the root canal (ANOVA, p<0.001). There was no significant difference in the temperature elevation between the two irradiated power settings (F15 and F22). The temperature elevation without water was slightly higher than that in water, but no significant difference was identified between these two conditions. This study suggests that there is a possibility that the temperature at the root apex markedly rises during intracanal CO 2 laser irradiation and may produce thermal periodontal tissue injuries.