The study involves a behavioural intervention with social norms approaches to correct smoking misperceptions, reduce smoking susceptibility and change smoking-related behaviours in Hong Kong adolescents. A 2-arm stratified clustered randomised controlled trial (RCT) will be conducted to evaluate the effectiveness of the intervention. A total of 18 secondary schools will be randomly selected from the 5 regions in Hong Kong and invited to the RCT. The schools will then be randomly allocated to intervention or control group in the ratio of 1:1. Secondary 1 to 5 (US Grade 7 to 11) students will be invited. Students in both intervention and control group will complete a baseline survey, and the intervention group will receive the intervention after the baseline. Intervention materials includes posters and table tents at school, flyers to students, and online banner on school website. The intervention adopts social norms approaches, and will highlight the prevalence of adolescents who do not smoke and have no intention to smoke. It will last for 2 months. Both intervention and control groups will complete follow-up surveys at 3 and 9 months. The control group will receive the intervention materials after the RCT.