Introduction. In our country dental caries and its complication are very widespread, following more clinical and economical difficult. Dental caries is multifactor infectious disease and its one of the most principal causative agents is acid attack, producing by cariogenic bacteria in dental plaque. If we have to detect the risk group early, we can prevent dental caries and treat in lightly form. In press review, there are a lot of microbiological and colorimetric tests, determining caries activity and identifying caries condition previously (J.Tsubouchi, Ts.Shimono et all., 1995: B.Oyuntsetseg, Y.Okazaki et al., 2004). Since 2005, The improving oral health condition of schoolchildren project invented by Taiwan Dental Association is going in 6 secondary schools of country town. Therefore 6 dentists are working in schools by prevention. There is no research work concerning the comparison the caries activity of pupils of secondary schools. The aims of this study were to compare a caries activity of pupils of secondary schools. Method: The study subjects consisted of 75 pupils and the study carried out in 2007. A dental examination was done using by mouth mirror and explorer under natural light. Before and after the lunch, for each pupil plaque was collected by swabbing from the buccal surfaces of the maxillary teeth using a sterile cotton swab. Each plaque sample was put into the Cariostat medium (21 test, Morita Co., Japan) and incubated at 37 C for 48 hours. After incubation the colorimetric changes were classified into seven grades using the original four grade standard color sample. Then, the pupils were grouped by Cariostat score as low risk (CAT 0-1.5) and high risk (CAT 2.0-3.0). Results: 1. The caries prevalence and deft score of pupils of 76th school were 84.2% and 3.590.31, but 67th school 73.2% and 3.000.31, respectively (p=0.01). 2. As assessed using CAT21 test, for the 76th schools 36.1% of pupils had high risk and 63.9% low risk before lunch, but after lunch 52.8% of pupils had high risk and 47.2% low risk. However, 67th schools 35% high, 65% low, 40% high and 60% low risk, respectively (p=0.01) (p=0.05). Discussion: 1. We carried out this study on two secondary schools with same life style. The caries prevalence and deft score of pupils 67th school were lower than 76th, so that general dentist had always working by preventive purpose for last three years. 2. That plaque pH results showed more stability in 67th school than 76th, may be pupils of 67th started toothbrushing after lunch. Conclusions: This study results showed caries activity of pupils of 76th schools higher than 67th. So that to have a dentist at the secondary schools very important.