In the country, homosexual intercourse in general and male homosexual intercourse (men who have sex with men - MSM) in particular is still discriminated because the common social conception considers that it is not suitable with Vietnamese customs. The lack of knowledge of prevention HIV transmission of these subjects leads to the high risk of HIV infection. Therefore, the authors conducted this study to describe the knowledge and practice of prevention HIV/AIDS transmission in MSM group in Nam Dinh province in 2012. This was a descriptive epidemiological research carried out by crosssectional survey of 200 gay subjects answering the set of question about knowledge and practice of prevention HIV/AIDS transmission. The result showed that the age of MSM group in Nam Dinh province was mostly under 30 (86 percent), and the percentage of the age between15 and 20 was 17.5 percent. Percentage of secondary and high - school graduate was 75.0 percent. The rate of understanding about HIV/AIDS in this group was between 60.5 percent and 100 percent. However, the rate of full awareness of HIV transmission was only 24 percent. There was 99 percent of MSM objects considering that using condom during sex could prevent HIV infection. However, in fact there was only 48.5 percent occasional use and 44.5 percent regular use. 98.5 percent of subjects knowed where HIV testing could be done and only 6 percent of subjects had done voluntarily HIV testing. From the result of this research, the authors recommended to pay more attention to communicate the knowledge of prevenion HIV transmission among MSM group.