Objectives: The fluoride coating for caries prevention and strengthen enamel use NaF(sodium fluoride, Junsei Chemical Co., Ltd, Japan) 2% gel, SnF2(stannous fluoride, SIGMA-ALDRICH Gmbh, USA)8% gel and APF(acidulated phosphate fluoride, Sultan health care, USA) 1.23% gel. Methods: After put the enamel piece in these fluoride compound gel, we observed density level. And after measuring the vickers hardness, Got the following conclusions. Results: 1. After settling in the APF 1.23% during 6 days, we observed high density level of enamel surface using 250 magnification scanning microscope. The vacuum of surface packed (in) like sardines. 2. After settling in the APF 1.23% during 6 days, we observed reducing the space between the cluster of enamel surface using 50,000 magnification scanning microscope. 3. The vickers hardness change was very much on the all kinds of fluoride compound gel[2% NaF(sodium fluoride)gel, 8% SnF2(stannous fluoride) gel, 1.23% APF(acidulated phosphate fluoride)gel]. It`s all because of reducing the space between the cluster of enamel surface(p<0.001). Conclusions: The vickers hardness change was very much on the all kinds of fluoride compound. It`s all because of reducing the space between the cluster of enamel surface. (J Korean Soc Dent Hygiene 2010;10(4):757-764)