Abstract: Introduction: Influenza is highly transmitted disease and vaccination is the most effective way to prevent influenza. This research was designed to study the variation of serum antibody level among the subjects had already been vaccinated against influenza. Materials and Methods: This research is a descriptive-analytical study, which was carried out on 196 subjects who had influenza vaccination (influvac 2005/2006) and 200 subjects matched by the vaccinated subjects, by age. The subject's serums were prepared seven weeks after influenza vaccination, and the control group's serums were also prepared. The serum antibody level was determined by haemaglutination inhibition test. Results: The mean age of case group is 52.2±11 and control group 48.64±5.17.The antibody titre of 115 of Vaccinated group and 15 of control is less than 40 1 The mean antibody titer of vaccinated subjects and control group is 143.4 ± 10.89 and 18.34± 3.2, respectively. The difference is statistically significant (P value=0.000). Conclusion: The findings show that the mean titer of antibody in vaccinated and control group is statistically different. It means that the influenza vaccine had a good efficacy. Key words: Vaccination, Influenza, Gorgan.