Objective: Substance use, such as alcohol and tobacco, is becoming a critical problem in the area of health promotion and preventive programme. Furthermore, the onset of alcohol use has been becoming increasingly earlier. This study examined developmental trajectories of substance use among high school students in South Korea and effects of peer substance on such trajectories, and also examined the moderating effects of parent-adolescent relationship quality and parental monitoring in the link between peer substance use and adolescents' own substance use. Methods: This study used the Korea Youth Panel Survey (KYPS), a nationally-representative sample of 3,125 high school students. The participants were in the 10th grade when first recruited and assessed annually from 2005 to 2007. A latent growth curve model was used for the analysis. Results: Substance use in Korean high school students increased over time. Peer substance use was associated more strongly with the initial level of adolescent substance uses than parenting. Parent-adolescent relationship quality moderated the association between peer and adolescent substance use. Conclusion: The effect of peer substance use was most strongly associated with the initial level and growth of adolescents' own substance use. (Contains 3 figures and 3 tables.)