Abstract: Objective　The study was conducted to investigate the epidemiological factors related to the outbreak ofparatyphoid A in a school in order to provide evidence for the development of corresponding preventive and controlstrategies. Methods　The epidemic strength as well as temporal, regional and population distribution was described based onclinical and in hospital registration records and newly reported cases each day. Epidemiological analysis was performed tocompare the incidences in senior versus junior school students, school living students versus commuters and male versusfemale. Field health survey was also carried out to identify the cause of disease. A total of 44 affected students whosediagnosis was laboratory confirmed were enrolled in the case group in 1∶2 matches to the control group that included 88students of the same sex, class and living arrangement who had no history of fever since September. Results　A total of 122patients were reported form Sep. 20 to Oct. 23 in 2007, the attack rate being 8.2%. Four patients were complicated with toxichepatitis or nephritis, and other ones were milder. After isolation and treatment, all cases recovered and were dischargedwith no death cases. The attack rate of senior high school students was 10.6%, higher than that of junior high schoolstudents(6.7%). Having meal in restaurants around the school(OR：6.60, X2＝10.29, P=0.001) and having meal with otherclassmates(OR：4.93, X2＝5.11, P=0.020)of the past two weeks were the risk factors associated with this outbreak.Conclusion　Having meal in restaurants around the school in the past two weeks was the primary risk factor of this outbreak.Having meal with other classmates and close contact were the risk factors associated with prophase and anaphase human tohuman transmission and spread, respectively.