Hydroxyapatite has high affinity rate with organic substance in an oral cavity, therefore lately nano-sized hydroxyapatite is extensively researched to increase affinity to tooth and remineralization. The objective of this research is indirectly to investigate the effect of nanohydroxyapatite on remineralization using valuation about whiteness and bacterial contamination of the bovine tooth. We have designed in vitro test for this purpose. The bovine teeth are sliced into human teeth size then the baseline whiteness of total 60 teeth is measured and assigned into 3 groups by cross-matching according to the whiteness. These samples were embedded into resin mold and underwent mechanical brushing during fixed schedule. The whiteness and bacterial contamination of teeth are measured and compared among 3 groups. Each dentifrice used in the experiment is composed of B.S.I group (30% Calcium Carbonate, precipitated-CaCo3) with abrasion rate of 100, Test Ⅰgroup (15% Sodium Metaphosphate, insoluble-NaPo3), Test Ⅱ group (15% Nano-hydroxyapate-Ca10(PO4)6(OH)2. After 20,000 strokes of mechanical brushing with each group sample teeth whiteness is measured for 2nd time and then the sample teeth were submerged into the same solution with oral bacteria for 10 days. The teeth whiteness is measured again for 3rd time and the bacteria from the surface of teeth are cultivated on agar badge for 3 days. We have compared with average of whiteness before and after test in group and each other 3 group. The whiteness of teeth was maintained in test group II, however in other group the whiteness achieved through brushing was lost. Also, Group II had the smallest area of bacterial colony on agar badge (p<0.01). It is concluded that the dentifrice including the nano-hydroxyapatite is replaced with the organic substance in enamel resulting the increase in whiteness and the resistance to bacterial colonization, which leads to the increase the enamel remineralization in teeth and result to decrease in caries susceptibility.