Laser treatment on dentin has been considered a new method to bring better adhesive effect.The purpose of this study was to observe the ultrastructure of dentin after the treatment of Er:YAG laser by scanning electron microscopy（SEM）and to assess the possibility of laser treatment on dentin to improve the adhesive properties between tooth and restoration. A total of 8 freshly extracted sound human maxillary third molars were prepared according to occlusal veneer. Extracted teeth were treated by Er: YAG laser after tooth preparation. The ultrastructure of dentin was observed by SEM. They were divided into 2 groups, 4 for the control group and 4 for the experimental group. After tooth preparation, the dentin of the experimental group was treated by Er:YAG laser. The specimen were observed by SEM including dentinal tubules, melting and fissure etc. It was observed that in the experiment group there was no smear layer, and the dentinal tubule was clear. Meanwhile, in the control group, there was obvious smear layer in the dentinal tubule therefore the dentinal tubule was not clear. This study suggests that Er:YAG laser treatment on dentin after tooth preparation can remove smear layer in the dentinal tubules, which may improve the adhesive properties between tooth and restoration.