The aim of the study is to evaluate the ablation threshold of enamel of human permanent tooth for Er:YAG laser and Er,Cr:YSGG laser. 70 enamel samples were treated with increasing energy densities of 2~10 J/cm2. The surface changes were assessed by means of reflected light microscope and scanning electron microscope. Both Er:YAG laser and Er,Cr:YSGG laser are effective in ablating human tooth enamel. The ablation threshold value of Er:YAG laser in dental enamel is 3.19~4.36 J/cm2. As to Er,Cr:YSGG laser,the ablation threshold value is 4.12~4.80 J/cm2 .