Balance is a complex concept, especially when one wants to do complex motor skills. Therefore, the balance during activity is more important than the static and dynamic balance that is performed without the skill. The aim of this study was to introduce a new activity-based balance index of elite soccer players. 27 young soccer players (12-14 years) whom participated in Iranian premier league were selected. To assess their balance, activity and skill, four functional tests consisting one leg stance, dynamic Y balance, running and dribbling tests were administered. During these tests, besides their relative scores, the acceleration of body center was also recorded using an accelerometer and a new activity-based balance index (ABI) were calculated from these available data. The Results showed a significant positive correlation between ABI and both static and dynamic balance scores(r=0.6, p=0.002). Also, a significant negative correlation was found between ABI and dribbling scores(r=-0.45, p=0.026), which could be interpreted as the necessity of agility of the players. It could be concluded that this new activity-based balance index was found to be successful for evaluating both the balance and the skill level of soccer players.