Objective Ceramic brackets debonding by Nd：YAG laser is based on the thermal effect of laser, which may cause injury of the pulp tissue. In this study, the histological changes of pulp tissue that subjected to Nd：YAG laser irradiation with different power and time were observed. Methods 20 New Zealand rabbits were included in this study. Ceramic brackets were bonded to the 4 incisors as routine. The ceramic brackets of left upper teeth that debonded mechanically were used as control group, while the brackets of right upper, left lower and right lower incisors were debonded by laser with 3 W 3 s（group A）, 2 W 5 s（group B） and 5 W 2 s（group C） energies, respectively. The teeth were pulled out at 5 minutes, 1 day, 3 days, 1 week and 1 month after the debonding operations. Slides prepared from the pulp tissues of the debonded teeth were used to evaluate the injury of laser. Results In comparison with the control group, pulp tissue of teeth that exposed to laser with different energy for 5 minutes showed mild capillary dilation. One day later, group A, B and C showed moderate capillary dilation, and group C also showed moderate infiltration. At 3 days, inflammation was disappeared in group B, whereas capillary dilation was found in group A. Hemorrhage and inflammation cells infiltration were found in group C. At 1 week, alleviation of capillary dilation was found in group A but not in group C. One month later, inflammation disappeared in group A, while pulp tissue in group C showed mild edema and capillary dilation. Conclusion Nd：YAG laser of high energy may cause injury of the pulp tissue during debonding. Laser energy of 3 W 3 s could effectively debond ceramic brackets without irreversible pulp injury.
