Objective:To evaluate the effects of Er,Cr:YSGG laser on the root surface of periodontally involved teeth and healthy teeth,concerning the microstructure and the roughness.Methods:Eight freshly extracted teeth due to severe periodontitis and eight freshly extracted teeth due to orthodontic rea-sons or being third molar were chosen in this study.The root surface of each tooth was divided into four areas,and received four treatment methods:no treatment (control group );root planing with Gracey scaler for 30 seconds;irradiation by the lower power Er,Cr:YSGG laser;irradiation by the higher power Er,Cr:YSGG laser.Four periodontally involved teeth and four healthy teeth were used for the evaluation of microstructure using scanning electron microscope (SEM).The other four periodontitis teeth and four healthy teeth were used for the evaluation of roughness (Ra value )using 3 D profiler.Results:Smear layer was found on the teeth scaled by Gracey scaler,while the teeth irradiated by Er,Cr:YSGG laser demonstrated a melting surface with less smear layer.In the periodontitis teeth irradiated by the higher power,opening dentinal tubules could be observed.For the periodontally involved teeth,the Ra values of groups 1 to 4 were (237.4 ±20.0)nm,(1 35.7 ±1 1 .9)nm (P=0.01 ),(463.6 ±49.3)nm (P<0.001 )and (486.0 ±59.0)nm (P<0.001 )respectively.For the healthy teeth,the Ra values of groups 1 to 4 were (1 91 .4 ±44.5)nm,(1 31 .6 ±21 .5)nm (P=0.482),(463.7 ±34.6)nm (P<0.001 )and (470.3 ±1 21 .3)nm (P<0.001 )respectively.Conclusion:Er,Cr:YSGG laser can affect the microstructure of the cementum of the periodontitis teeth and healthy teeth.Irradiation by the Er,Cr:YSGG laser resulted in a melting surface with less smear layer and increased the roughness in the surface of root.