ABESTRACT: Introduction & Objective: The efficacy of hepatitis B vaccine and duration of protection after vaccination of infants reported to be different in various studies. The necessity of booster dose after primary vaccination is controversial. The Iranian Immunization Committee has approved 0, 1 and 6 months hepatitis vaccination schedule for children. Considering the high prevalence and serious outcome of hepatitis B infection, the aim of the present study was to evaluate the immunity level of school age children against HBV in order to determine the necessity of single booster dose in these ages. Materials & Methods: In this clinical trial study, the population was all of the children at 5-7 years of age in Kohgiloyeh & Boyerahmad province who had been vaccinated starting at birth with hepatitis B vaccine. Among them 800 children were selected by multiple stage sampling method. Data gathering tool was questionnaire. After obtaining informed consent from parents of each subject, 3ml blood sample was taken from each individual and hepatitis B surface antibody (HBS-Ab) and hepatitis B surface antigen (HBS-Ag) were determined by ELISA method. Subjects with non protective titers (<10mIU/ml) received a booster does of DNA recombinant vaccine. Four weeks after the administration of booster dose, the anti- HBS titers were measured. Collected data were analyzed using SPSS software and x2 and T tests. Results: In all subjects the HBS- Ag were negative. 84.4% of subjects were immune against HBV (had protective anti body titer). The mean antibody titer was 230.5 ±308.9 IU/ml with range 10.6 – 1175 IU/ml. 15.6% of subjects had non protective antibody titer and mean antibody titer was 4.97 ±3.5 IU/ml. Following booster dose 78.1% of them had protective titer and significantly increased titers from 4.97 ±3.5 to 332.1 ± 402 IU/ml (p<0.001). No statistically significant differences were found between sexes in term of antibody level. The level of immunity against HBV was found to decrease with age. Conclusion: According to our results, the proper response of the immune system to booster dose shows that the immunological memory is good after primary vaccination. Administration of booster dose would not be necessary in these ages despite the possibility of more exposure of children in school age. Further study about using booster in older ages and children who does adequately respond to primary vaccination is necessary. Keywords: Vaccination, Booster ose, Hepatitis B, Efficacy