Abstract: This cross sectional study was performed to estimate the prevalence of serological evidence of immunity to Measles in primary school students (age 6-12) that have a history of twice vaccination against measles, on 9 and 15 month old and determining the appropriate age to re-vaccination. Multistage sampling was used to select students to participate in the present study. Immunization status and documentary evidence of immunization was recorded from the Personal Health Record. Measles antibodies were measured using enzyme-linked immunosorbent assay (ELISA). Sufficient blood for antibody testing was obtained from 1665 children, 975 (57%) were girls and 720 (43%) were boys, respectively and mean age was 9.17±1.53 years. On the whole, 1198 subjects (72%) were seropositive and 467 (28%) were seronegative at all. Among girls and boys, 72 and 71%, were seropositive respectively. These differences were not statistically significant between the two sexes (p = 0.404). Antimeasles antibody titer decreased with increasing age from 6 to 10 years old, (76.5% in 6 years old group vs. 71% in 10 years old group) and then rise to 78% in 11-12 years old group. These differences were not statistically significant between age groups (p = 0.775). the immunity produced by the measles vaccine is not enough. Decrease of protective effect of measles vaccine, suggests the necessary of pre-school revaccination.