Several studies have reported that physical inactivity and sedentary lifestyle are associated with being overweight and obese in children and adults. A new policy of 1-h physical activity (PA) every day was released by the Chinese government. The present study examined the role of 1-h PA every day in preventing obesity in adolescents in Shandong, China. A total of 29,030 students (14,578 boys and 14,452 girls) aged 10-18 years participated in this study. Height, weight, waist circumference (WC), and skinfold thickness (SFT) of all subjects were measured; body mass index (BMI) of adolescents was calculated from their height and weight, and the prevalence of overweight and obesity was obtained according to the International Obesity Task Force cutoffs. All subjects were divided into two groups. Group 1 had a PA of more than 1 h/day while group 2 had less than 1 h/day. Comparisons of BMI, WC, SFT, and prevalences of overweight and obesity between the two groups were made. The overall percentages of students in group 1 were 34.29 % in boys and 30.15 % in girls. The prevalences of overweight and obesity for both boys and girls were all significantly lower in group 1 than in group 2 in all age categories. In conclusion, 1-h PA every day has a beneficial effect in preventing obesity in adolescents in Shandong, China. These observations highlight the importance of PA in the prevention of overweight and obesity in adolescents.