Hepatitis A antibody prevalence in Southeast Asia has markedly declined among children and adolescents. Therefore increasing a number of susceptible populations could result in an outbreak or epidemic. This paper reports an intensive study of an outbreak of hepatitis A in a primary school children during an endemic at Nakhon Si Thammarat, Southern Thailand. Eighty-nine children were inspected randomly from the total of 269 students, age ranged from 7-12 years old. The school children and parents were interviewed for their illness. Serological tests for antiHAV IgM and antiHAV (total antibody) were performed by ELISA. There were 36 cases of clinical acute hepatitis were positive for antiHAV IgM because of serological tests were performed nearly 4 months later. Seventy of 89 children tested were positive for antiHAV and 16 of them were positive for IgM class. Seven of 16 children with antiHAV IgM positive were asymptomatic. The significant risk factors for children with positive antiHAV were occurrence of hepatitis patients in the family and no latrine (p < 0.01). Endemic transmission in this outbreak occurred rapidly. Therefore preventive measures are essential in reducing the infection rate. In addition to personal hygiene, immunoprophylaxis with either immunoglobulin or HAV vaccine is recommended.