Abstract: Objective The present study was conducted to find out the status of immunity against diphtheria after DPT(diphtheria-pertussis-tetanus vaccine)vaccination in children aged from 1 to 7 in Yuxi City of Yunnan Province to provide a scientific basis for the development of strategies for anti-diphtheria immunization.Methods Indirect hemagglutination assay(IHA)was used to detect the antitoxins in the serum samples collected from 1937 children aged 1-7 who had had DPT vaccination from 2003 to 2005.Results The overall positive rate of diphtheria immune antibodies was 94.84% with the protection rate being 90.76%,the geometric mean titer(GMT)1∶52.23,and the mean antitoxin content(ATC)0.15IU/ml.There was no significant difference found in the positive rate of diphtheria immune antibodies between the boys and the girls(X2=2,P0.05),while their difference in the mean titer and protection rate were statistically significant(u=3.3,X2=7.87,both P0.05).There were statistically significant difference in the positive rate,protection rate and GMT between different areas(X2=7.75,X22=14.1,F=43.2,both P0.05),age groups(X12=21.59,X22=27.1,F=7.32,both P0.05)and years(X12=9,X22=37.1,F=22.21,both P0.05).Conclusion The quality of DPT vaccination is well controlled in Yuxi city,and a high immune antibody status in children aged 1-7 and a solid immune barrier have been maintained.It's predicted that a diphtheria epidemic is not likely to occur in the city in coming years.