Aim: To study the effect of 7-valent pneumococcal conjugate vaccine ( PCV7), pneumococcal nasopharyngeal carriage was compared between vaccinated children and controls. Method: Nasopharyngeal (NP) swabs were obtained from a total of 400 healthy children aged 18-59 months in Seoul, Incheon and Jeju Island. Among them, 200 children fully vaccinated with the 3+1 doses of PCV7 vaccine were enrolled in the vaccinated group and 200 children with no vaccination history of PCV7 were enrolled in the control group. NP swabs were obtained from June to December, 2008 and cultured for S. pneumoniae. For each detected isolate, the serotype and antibiotic susceptibility were determined. Serotyping was done by the quelling reaction (SSI, Denmark) and also the multibead assay (UAB, USA). Antibiotic susceptibility was determined for penicillin, amoxicillin, erythromycin and cefaclor. Results: Pneumococcal carriage rate was 15.5% (31/200) for the vaccinated group and 31.5% (63/200) for the control group. Among the vaccinated group, 48.4% (15/31) of the serotypes were vaccine-related type (VRT: 6A, 6C, 19A) with the most common serotype 6C. In contrast, a majority of the isolates in the control group were VT {VT serotypes in vaccine vs. control; 12.9% (4/31) vs 52.4% (33/63), P0.05}. According to the antibiotics susceptibility analysis, regardless of serotype, more strains were penicillin and erythromycin susceptible in the vaccine group (vaccine vs. control; penicillin 22.6% vs 3.2%, erythromycin 25.8 vs 9.5%, P0.05). Also, among all nonvaccine type serotypes (NVT, including VRT and nontypable), the proportion of penicillin susceptible strains were higher in the vaccine group (vaccine vs. control; 25.9% % vs 3.3%, P0.05). Conclusion: Compared with the control group, PCV7 vaccine reduces carriage in VT serotypes which may lead to replacement of pneumococci by antibiotic susceptible NVT strains.