Pandemic (H1N1) 2009 virus outbreak occurred in Vietnam since late of May, 2009. This study aims at determining attack rate in Pandemic (H1N1) 2009 virus outbreaks in schools, army base, community. Retrospective studies of case investigations were implemented in Ngo Thoi Nhiem school, cluster in a party in Xuan Loc district. Prospective studies were implemented in 4 schools in Ben Tre province, an army base, a police base. Study subjects included the whole risk population of community with confirmed cases. Attack rate was defined as percentage of cases with positive result of RT-PCR. Study showed that the highest attack rate is in family party (80.6), following are army base (25 percent), school with dormitory (7.1 percent), school without dormitory (1.11 - 3.13 percent).