Abstract: Objective To provide the evidence for prevention and control of severe case of novel influenza A(H1N1)through analyzing epidemiological characteristics and risk factors of severe infection in Yunnan.Methods We describede pidemiological characteristics of severe cases of novel influenza A(H1N1) and conducted a 1:1 matched case control study and compared the mean time of onset to first visit and mean time of onset to confirm ation bewteen severe cases and non severe cases.Results Twelve prefectures reported severe cases among 16 prefectures in Yunnan.Most of severecases(87.39%) were reported in Qujing and Kunming city.Students and peasants were at a higher risk of getting infected among non severe cases.Whereas 70-75 age group, which were mostly retired citizens, was at a higher risk of severe infection.Non severe cases had shorter mean time of onset to firstvisit and mean time of onset to confirmation; the difference between non severe and severe cases was statistically significant(P < 0.05).Conclusion The number of severe cases of novel influenza A(H1N1) peaked in December 2009.Most of severe cases were detected in Qujing and Kunming city.The eldely over 70 years and retired citizens were the populations at higher risk.