In most existing performance evaluations of ad hoc routing protocols, connectivi ty is not considered properly. To solve this problem, a new concept of network coverage density is adopted, which reflects communication ability of nodes and connectivity of ad hoc network. Based on this, a research is conducted on the rela tionship between performance of topology-based MANET routing protocols and netwo rk coverage density. Simulation results show that, packet delivery of routing pr otocol heightens with the increase of network coverage density, and they form an S curve. Through the definition of minimal network coverage density, a judgment criterion for applicability of routing protocol is provided in the process of a d hoc networking.