The objective of this study was to examine the influences of social network factors, particularly social support and norms, in the transition from non-injection heroin and/or opiate use to heroin-injection, which is one of the leading causes of the spread of HIV/AIDS in China. Respondent-driven sampling was used to recruit young heroin and/or opiate users in an egocentric network study in Yunnan, China. Multivariate logistic regression using hierarchical combinations of candidate variables was used to analyze network factors for the injection transition. A total of 3,121 social network alters were reported by 403 egos with an average network size of eight. Fifty-eight percent of egos transitioned to heroin-injection from non-injection. This transition was associated with having a larger sex network size, a larger number of heroin injectors in one's network, and a higher network density. The findings enhance our understanding of the influence of social network dimensions on the transition to injection drug use. Accordingly, the development of interventions for heroin and/or opiate users in China should consider social network characteristics.