Criminals have high risks of acquiring HIV, accounting for more than 1/10 among HIV cases in Vietnam in 1998. It is necessary to evaluate knowledge in HIV/AIDS prevention, from which interventions can be pointed out to reduce HIV/AIDS transmission. Therefore, the authors performed this crosssectional research on 400 criminals at Dien Bien prison in 2009. The result showed that: 73.7 percent of criminals acknowledged that HIV transmit through blood
68.7 percent of them knew that HIV transmit through sexual intercourse 58.5 percent of those acknowledged that HIV transmit from mother to child. 48 percent of criminals supposed that there has not been cure for AIDS
42,3 percent of criminals had no idea about treatment for AIDS. 67.5 percent of criminals had knowledge in HIV exposure, 62.7 percent of criminals knew how to manage in case of HIV exposure.