For studying on the maximum damage probability of anti-UAV by air defense rocket with an optimal detonation distance, new probability models were developed to examine the potential lethality. UAV's vulnerability, susceptibility and penetrator spray error were analyzed, and damage probability assessment model was constructed via a vulnerability area reference coordinate system. The result shows that these models are used to determine the detonation distance which maximizes the probability that a single hypervelocity rocket kills a UAV. Data produced in this study provides an optimal detonation distance for fire control system.