The purpose of this study were to determine the physical activity level and its affecting factors of junior students in Taipei City. The physcial fitness, social support, self-efficacy and physical fitness knowledge classified by 3 varied degree of physcial activity level of each subjects, were classified to compare the difference. In addition, the related variables were used to predict the physical activity levels. Totally 676 junior students from 8 junior schools in Taipei City were stratified and chosen randomly ad subjects. Each subject was asked to fill out the designed questionnaires of the related variables and received a battery of physical fitness test. The physical fitness test items included: 1) body mass index, 2) one-minute sit-up, 3) sit and reach, 4) 1600m (male) or 800m (female) run-walk test. The data were analyzed by one-way ANOVA and Step-wise linear regression, the results were followed: 1. The sex and age of the subjects have a significant effect on physical activity. 2. The junior students involved more in moderate-vigorous physical activities on weekend leisure time than that of weekday. The more popular activities were playing baseball, soft baseball, volley ball, table tennis, ten pins bowling, and riding bicycle. 3. The moderate-vigorous physical activity of junior students had significant relationship with social support, self-efficacy and physical fitness. 4. The 23.8% variance of physical activity could be explained by sex, age, self-efficacy, social support and regular exercise habits. 5. The self-efficacy, social support, muscular endurance, cardiovascular endurance of high activity ones. The self-efficacy, social support, cardiovascular endurance of middle physical activity subjects were significantly better than that of low physical activity ones.