BackgroundThe aim of this study was to evaluate the effect of combining fractional CO2 LASER and nanohydroxy apatite on surface microhardness and color of enamel with initial defects.Material and MethodsTwo types of nano hydroxylapatite (nHAP) was prepared; Pure hydroxyapatite (nHA) and Fluoro hydroxyapatite (nFHA), Sixty extracted premolar teeth without visible caries or structural defects on enamel surface were used, immersed in 10 ml of a demineralizing solution for 2 weeks to create artificial white spot lesions, they were randomly allocated into two groups; Group 1: nHA, Group 2: nFHA, each group is then subdivided into 2 subgroups (A and B) where two different in vitroremineralization procedures have been performed, the first procedure utilizes a 10 wt% nHA aqueous slurries only, the second was first exposed to irradiation from a fractional CO2 laser then (nHAP) was applied. Microhardness and color were measured using a micro-Vickers hardness tester and spectrophotometer respectively.ResultsLaser treated teeth in both groups showed the highest mean hardness and lowest color difference where ΔE was less than 3.3 units, in both tests the pure type of nanohydroxyapatite gave better results than the nanofluroapatite type.ConclusionsNano-hydroxyapatite has remarkable remineralizing effects on initial lesions of enamel, certainly higher when combined with laser application. Key words:CO2 LASER, Enamel remineralization, Nanohydroxy apatite.