The purpose of this study was to examine the effectiveness of an Internet-based behavioral intervention for physical activity (PA) promotion among secondary school students. It was hypothesized that the Internet-based PA promotion program could increase the PA levels of secondary school students. The action research approach together with qualitative focus group interview was used. A total of 127 Form 4 (Grade 10) students from a Hong Kong Secondary School were recruited in the study. They were group-randomized by class and placed into two groups--the Internet-based group (IB) and the control group (C). Participants of the IB group received the online behavior change program entitled "let's exercise and be active daily" (LEAD) for PA promotion among secondary school students. The international physical activity questionnaire short last 7-day instrument was used to assess the PA levels of the participants. The stage of exercise change questionnaire was used to assess the stages of behavioral changes of the participants. The assessments were conducted at baseline and post intervention both after 1st cycle and 2nd cycle. The narrative approach of data analysis was selected in the focus group interview after two cycles of data collection. There was a trend for the participants of the IB group to participate in more PA at the 1st cycle and 2nd cycle assessments, but no statistically significant differences within the IB group or between the study groups were observed. Limitations and suggestions were made in the reflection of the overall study.