Objective: To elucidate age and sex specific characteristics in fat free mass (FFM) among middle and high school adolescents in Yinchuan City, China, and to provide a scientific basis for healthy development, chronic disease prevention and intervention in children and adolescents. Methods: Using a convenient cluster sampling method, 1 175 middle and high school adolescents, aged 10 to 20 years old, from Yinchuan, China, were selected to participate in a questionnaire survey and physical examination, which involved body composition by bioelectrical impedance analysis(BIA). Results: For adolescents aged 12 to 18 years, FFM and the fat free mass percentage (FFMP) of boys increased with age(from 39.78 to 52.63 kg; 77.51% to 83.80%, respectively), the age trends of the FFM and FFMP of body parts were in the similar pattern, and were significantly higher than those of girls(all P <0.05). In girls, the FFM reached a peak of 40.96 kg at 15 years old, and decreased slightly between the ages of 15 and 18 years, the age trends of the FFM of body parts were in the similar pattern, and the FFMP decreased with age (from 75.63% to 71.91 %). In addition, in girls, the age trends of the FFMP of left and right legs were the same as the general trend, while the FFMP of left and right arms increased with age. The FFMP of the trunk reached a minimum of 29.93% at 15 years old and increased from 15 to 18 years old(all P <0.05). Conclusion For middle and high school adolescents aged 12 to 18 years old in Yinchuan City, China, the distribution of FFM changed in accordance with age and gender differences, in accordance with the characteristics of the adolescents growth and development.