This study aimed to find out the correlation between body mass index (BMI) with the health-related fitness and motor performance-related fitness of people with Down syndrome in the governorate of Muscat aged between (15-18 years), and to identify the differences in the level of health-related fitness and motor performance-related fitness elements between the two age group (15-16) and (17-18) years. Descriptive research methodology was used for its suitability to the study's objectives. The study sample consisted of 20 students with Down syndrome, were selected purposively of students enrolled in the School of intellectual Education and merger schools affiliated to the Ministry of Education in the Governorate of Muscat, measured their extraction of the body mass index and health-related 'fitness elements: (cardio respiratory fitness,' musculoskeletal strength, flexibility, muscular endurance and Body composition), and motor performance-related fitness elements: (balance, muscular power, speed, and agility). The results indicated a significant correlation between BMI and cardio respiratory fitness, abdominal muscles strength, within health-related fitness elements, and speed, agility, and trunk flexibility, within motor performance-related fitness, and lack of statistically significant differences in health-related fitness elements and motor performance-related fitness elements between age groups (15-16) and (17-18) years. Therefore the researcher recommends to give more attention by the competent authorities as well as setting curriculum and sports specialized programs contribute to raising the physical level for the Down syndrome students, Finally, similar studies and research are to be carried in other Down syndrome different age groups,