Infant milk formulas may cause tooth destruction by producing different organic acids after fermentation by oral microorganisms. The aim of this study was to investigate pH changes in children’s plaquesaliva solutions after fermenting four commonly used infant milk formulas. Plaque saliva specimens of 25 children were sampled. Infant milk formulas including Bebelac 1, Nan 1, Nan 2 and Aptamil 2 were prepared and added to four respective tubes of saliva-plaque solutions. The pH of samples were measured at 10 min intervals (10-60) at 37°C. All infant milk formulas caused significant pH drop in all of the samples. Aptamil 2 and Bebelac 1 showed nearly the same pH drop and caused more pH value decrement than Nan 1 and Nan 2 after 30, and 60 min. The difference in pH decrement in solutions may be related to their different amounts of fat content as a preventive factor in acid production, as the amounts of other ingredients such as carbohydrate and protein were nearly the same.