Objective:To detect the caries activity of orthodontical treated and untreated adult.Methods:The study population consisted of 30 adults whose age are over twenty,the control group includes 30 adults who did not accept any orthodontical treatment.The detective methods were enumeration of streptococcus mutans in saliva,ability of plaque to produce acid and salivary buffering capacity.Student's test was used to analyse the statistical difference.Results:The results showed that the number of S.mutans and lactobacillus on the untreated group are much lower than that of the orthodontical group;the salivary pH,salivary buffering capacity and ability of germ to produce acid are similar between orthodontical treated and untreated adult.Conclusion:This study suggests that the orthodontical treated adult are more sensitive to dental caries than untreated adult.