The present study was conducted to find out the relationship between aerobic power of an individual, its body mass index and its physical performance so as to improve the physical fitness of low body mass index subjects bringing their better performance. The study was carried out in 58 young healthy sedentary medical students who were asked to perform short term limited duration exercise on bicycle ergograph. Each subject served as its own control. Subjects were introduced a pretested questionnaire for assessing nutritional status, Cardiorespiratory profile, status of nervous system , physical activity and related problems, history of past and present illness to find out any condition affecting physical performance of the subject. Physical activity level was assessed by different tests assessing their flexibility, coordination, Equilibrium, agility, strength and endurance. Physical activity Rating scale and VO2 max was used to assess the physical fitness. Pearson’s correlation coefficient was to determine the relation between BMI and VO2max.The observation revealed a positive correlation between physical fitness score and normal BMI value and negative with the value which is higher and lower than normal. A significant positive correlation was also found between BMI and Post exercise VO2 max. A positive correlation was also observed between Physical fitness index and Vo2 max. t could thus be concluded that the study population having normal Body mass Index showed greater improvement in physical capacity as compared to subjects having high or low BMI. The study also revealed that aerobic capacity improves with training.