Study was conducted in the laboratory of Biological control of Cantho University to determine the caused agent of soft rot disease on onion based on morphological and biological characteristics. Infected samples were collected on onion fields of Vinh Chau, Soc Trang province. The bacteria were isolated on Water Agar (WA) and King's B agarmedia. Bacterium indentification follwing the Loch's postulate and based oncolony, bacterial shape. Gram, flagella characterictics, biochamistry, reaction, ability ofanaecrobic, reaction of catalase, indole and gelatine. Results showed that the bacterium Erwinnia carotovora is the caused agent of soft rot disease on onion at Vinh Chau.