Objective To investigate the effect of pulse Nd:YAG laser combined with NaF on caries resistance of human enamelin vitro.Methods 24 caries-free, constant premolares were cut into enamel mass. Surface ultrastructure and fluoride distribution of enamel in laser-NaFtreated group, lasertreated group, NaFtreated group and control groupwere observed by scan- ning electron microscope (SEM) and electron probe microanalyzer.Results ①SEMobservation: In laser-NaFtreated group, the smear layer was almost removed, the enamel surface showed partially melted-like and bubble-like appearance, diffuse white-small particles distributed on enamel surface, and melted position, microcrack and microhole appeared whitely blocking-reflection. ②Electron probe microanalyzer analysis: In enamel samples, the highest average Ci of F-and the hightest depth which F-en- tered in enamel were recorded in the laser-NaF treated group. The average Ci of F-in laser-NaF treated group was significantly increased compared with that in NaF-treated group or laser-treated group or control group (P
