Abstract: Objective To understand the immunity levels of pertussis, diphtheria and tetanus among healthy people aged＜30 years in Nanjing, evaluate the effect of DPT vaccination and provide scientific evidence for the development of vaccination protocol and monitoring of vaccination effect. Methods The serum samples were collected from 600 healthy people aged＜30 years to detect pertussis antibody level by micro agglutination test and antibody level of diphtheria and tetanus by indirect hemagglutination (IHA). Results The positive rate and protective rate of antibody to pertussis was 85.1% and 24.3% respectively with the geometric mean titer (GMT) of 1 ∶ 69.23, the positive rate and protective rate of antibody to diphtheria was 86.9% and 76.1%, respectively with the GMT of 1 ∶ 38.90, and the positive rate and protective rate of antibody to tetanus was 85.9% and 78.4% respectively with GMT of 1 ∶ 48.92. The difference on positive rate of antibody to pertussis in all age groups was not significant statistically, but the differences on protective rate and GMT were significant statistically. The differences on positive rates, protective rates and GMTs of antibodies to diphtheria and tetanus in all age groups were statistical significant. Conclusion Herd immunity to pertussis, diphtheria and tetanus had formed among healthy people aged＜30 years in Nanjing, but the GMTs declined significantly among people aged＞15 years. It is necessary to strengthen the prevention and control of pertussis, diphtheria and tetanus in older age group