In order to study the relationship between cluster formation and radius, the Cucker-Smale model with a hierarchical structure of a single leader and the leader speed invariable was considered. The influence of free will on cluster was discussed. The sufficient conditions that the radius has a lower bound (the lower bound was related to the velocity difference, particle number, communication intensity, etc.) were obtained by proving. When the radius was greater than the lower bound, the cluster would be generated. The related conclusion was verified by MATLAB numerical simulation.