On 16 Oct 2009, a provincial health officer notified to the Thailand Bureau of Epidemiology that 50 Army Reserve Force Students (ARFS) from a two-week training camp in a northern province received treatment at a hospital for diarrhea in the past two days. An outbreak investigation was initiated to verify diagnosis, identify risk factors and recommend control measures. We reviewed medical records and interviewed all camp participants to identify ARFS with diarrhea. A retrospective cohort study was conducted to identify risk factors. A total of 257 diarrhea cases were identified from 470 people at the camp, including 256 ARFS (AR=57%) and one trainer (AR=17%). Common symptoms included abdominal pain (85%), loose stool (83%), fever (63%) and watery diarrhea (59%). Green chicken curry in coconut milk served at dinner on 12 Oct 2009 might be a risk factor (Adjusted odds ratio=4.5, 95% confidence interval=0.5, 42.1). No food or raw materials of the suspected meal was left for laboratory testing. Rectal swabs from seven patients and four food handlers, including the cook who prepared the suspected meal, were tested positive for Salmonella serogroup B. The outbreak suggested a common source. Food sanitation, particularly health screening for food handlers, should be emphasized for mass gathering.