Objecive To identify the source of infection,route of transmission and risk factors of a cluster of acute gastroenteritis cases in a primary school of Guangzhou.Methods Cases were identified according to the case definition.Descriptive epidemiological approaches and case-control study designs were employed for the analysis.The samples were tested for norovirus by RT-PCR.Results A total of 27 cases of norovirus gastroenteritis were identified in March 28 to 31,2013 and the attack rate was 3.31%(27/816).The peak in morbidity was seen on March 28 at 14:00 to 22:00.Clustering was found in classes.The case-control study revealed that washing hands before meals(OR=0.13,95%CI:0.03~0.63) and wash hands more than 5 times a day(OR=0.26,95%CI:0.10~0.72) were protective factors.A total of 85 samples,including rectal swabs from the patients and Kitchen workers,leftovers and environmental swabs were collected.Five samples were positive for norovirus G Ⅱ detected by RT-PCR.Conclusion This is the acute gastroenteritis outbreak caused by the norovirus genotype II-4.It was a foodborne outbreak in which food was contaminated by the asymptomatic kitchen workers who carried the virus.