Abstract: An outbreak of influenza B occurred in a primary school in Pingxiang from February 2 to March 3,2012,involving 98 cases of influenza like illness (ILI) in 23 classes, the attack rate (AR) was 4.3%. The cases mainly occurred in class 2 and 4 of grade 2, the AR was 55% in class 4 and 30% in class 2. All the cases had fever, 79% of the cases had cough and 77% had sore throat. Retrospective cohort study indicated that the influenza vaccination rate was only 18%, which was quite low; The AR was 17% among vaccinated students and 49% among unvaccinated students, the influenza vaccination protection rate was 66 % (95% CI: 2.0%-88.0%). Students with hand washing habit were less likely to be infected.