-Research sample: We based on the formula of sample size calculation Where: n1=sample size for the intervention group (use 0.05% fluorideoral rinsing solution), n2=sample size for control group (used 0.05% fluorideP / S toothpaste for children); Z (1-α/2) = the reliability coefficient, kept in 95% (=1.96); Z1-β=power (= 80%); p1= the rate of incipient caries in intervention group after 18 months follow up is estimated 40%; p2 = the rate of incipient caries in the control group after 18 months follow up is estimated 55%; p = (p1 + p2) / 2. According to the formula, the minimum required sample size for both of research groups is n1 = n2 = 168 pupils.