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Abstract Introduction According to Behavioral and Biological Integrated Surveilance (BBIS) in Indonesia, 2011, there was an increase in syphilis surveilance in men who like to commit sexual intercource with other men (MSM). It was 13% of the 3% in BBIS 2007 in bad STI prevention behavior . There were 478 MSM have visited STI-VCT clinic in Medan throughout 2015 and 59 men were infected by syphilis. Objective The  aim of this study is to analyse the influence of perception and peer support on prevention of STI in MSM at Veteran STI-VCT Clinic in Medan, 2016. Methods This was a mixed methode quantitative and qualitative study with cross sectional approach, enrolled 50 respondens. Data was collected and analyzed with SPSS 19. Results There was  influence of perception and peer support on STI prevetion behavior of MSM group at STI-VCT Veteran Clinic in Medan Keywords: perception, peer support, prevention STI, MSM