The recent research by a group of European scientists shows that, each starling bird adjusts its flight in the light of topological interaction, that according to a fixed number of its nearest neighbors, rather than all agents within a fixed Euclidean distance. A theoretical study to understand such topological interaction is given in this paper, and a sufficient synchronization condition imposing only on the initial states is developed. Furthermore, the number of neighbors needed for the synchronization is discussed, which shows that for the sake of connectivity the number of neighbors should increase as the density of group increasing. Keywords: network topologies, distributed non-linear elements, synchronization.