Objective: To analyze the development status and sensitive periods of Han and Mongolian students physical fitness development in the Inner Mongolia Autonomous Region, to provide a basis for further research on the growth and development of young students and the selection of athletes. Methods: In 2014, 15 370 Han and Mongolian students from the Inner Mongolia Autonomous Region, ranging in age from 7 to 18 years old, participated in this study. Data on six physical fitness indices, namely grip strength, 50 meters, standing long jump, 800 meters or 1 000 meters, pull-ups/sit-ups in succession, and sit-and-reach were collected. Furthermore, using the average annual growth rate and the standard deviation of half annual growth rate, critical values and the ages at which sensitive periods occur were calculated to examine the differences in the development of various physical qualities between the two ethnic groups. Results: The students showed different growth rates of various indicators at different ages. A sensitive period for strength occurred in Han and Mongolian boys aged 13-15 years old and 12-15 years old, respectively. Sensitive periods for speed occurred in Han and Mongolian boys aged 8-10, 13, and 15 years and 8, 10, and 13-15 years, respectively. Sensitive periods for endurance occurred in Mongolian girls aged 8, 14, and 18 years. Sensitive periods for flexibility occurred in Han girls aged 13 and 15 years old. Conclusion Han and Mongolian students experience sensitive periods for different physical qualities at different ages. Moreover, there are apparent ethnic and gender differences that are retained across age groups and several scattered age points that exist alone or interspersed with each other.