Physical activity is the first line approach and one of the main factors in preventing chronic diseases. Currently there is the increasing percentage of sedentary life style or lack of exercise among adolescents. The main objectives of the study were to determine the prevalence of inactivity and the factors influencing physical activity in adolescents. A cross sectional study was carried out among secondary school students aged 14 and 16 in Petaling District, Selangor, Malaysia. A total of 519 respondents participated in this study. Their physical activity level was measured using the International Physical Activity Questionnaire (IPAQ). The active group was classified as those having levels of equal or more than 600 met-min per week while less than 600 met-min per week was considered inactive. Response rate in this study was 95.4%. The prevalence of inactive in adolescents was 20.8%. Female adolescents, non-working mother, time constraint, exercise only when having ample time and stretching before exercise are predictor factors for being inactive among adolescents. Steps need to be taken to persistently ensure that the physical activity among adolescents be increased continuously.