Abstract
Asteroid families are thought to be formed through collisional events that a catastrophic impact kicks on the parent body and produces many fragments ejected away. The ejection-velocities of these fragments might be related with their sizes. Through the observed and theoretically produced velocities of asteroids, we study the original ejection-velocities of fragments, and investigate the implied vleocity-size relation of asteroid families. Three models were proposed, Takagi model, Nakamura one-impact model and Nakamura two-impact model separately. In Vesta family, we found that maybe it didn't exist the velocity-size relation or most members were produced from low impact-velocity events of the collisional era.