Background: Musculoskeletal fitness and body composition are usable markers of individual’s total health. Adolescence represents a pivotal stage in development. Aim: To compare between different categories of body mass index (BMI) in muscle thickness and physical fitness in adolescents. Subjects and Methods: Cross sectional study included 100 adolescents aged from 12 to 18 years (42 girls and 58 boys) were categorised by BMI into 4 equal groups (overweight, obese, normal and underweight). Muscle thickness of biceps brachii, rectus abdominis and rectus femoris was measured by ultrasound imaging, and physical fitness was assessed by European physical fitness test battery. Results: There were significant differences between the 4 groups in tests of standing broad jump, sit-ups and bent arm hang tests (p=0.001) as normal adolescents were the best between groups in bent arm hang test and were better than obese and underweight in sit-ups test, while obese adolescents were the worst in sit-ups and standing broad jump tests compared to normal and overweight. Non-significant differences were found in tests of 10 x 5 shuttle run and plate tapping tests between the 4 groups (p=0.11, 0.09) respectively. There were significant differences (p=0.001) between the participating groups in muscle thickness. The underweight group had the lower muscle thickness for the three tested muscles compared to other groups, while the overweight and obese adolescents were higher in rectus abdominis and rectus femoris muscle thickness Conclusion: Obese adolescents are the worst in physical fitness tests performance; which emphasize the need for regular muscle strengthening and fitness activities for younger children and adolescents. It seems that individuals with a higher BMI have greater muscle thickness compared to their peers with normal or lower BMI