Abstract: Objective To analyze the epidemiological characteristics, related factors and infection source of a dengue fever outbreak in Yiwu. Methods The field epidemiological survey was conducted and the virus isolation from the patients was performed for virus sequencing. Results Totally 294 dengue fever cases (no death) were confirmed in Yiwu from 20 July to 4 October,2009, the cases were mainly occurred during late August and mid September in 4 villages and affected 16 villages. The time and spatial clustering features of the outbreak were obvious. The cases were with typical clinical manifestation of dengue fever. The sequencing of the isolates indicated that the outbreak was caused by dengue virus type 3, which might come from the South East Asia. Conclusion The outbreak was caused by the imported case. The introduction of virus from foreign country, high density of mosquito vector(Aedes albopictus) and missed diagnosis might be the major influence factors.