Abstract: Ad hoc On-demand Distance Vector (AODV), Dynamic Source Routing (DSR) and Temporally Ordered Routing Algorithm (TORA) are three on-demand routing protocols for ad hoc networks. This study described the principles of the three protocols. The theoretical models of nodes distribution are then introduced. Finally, the effectiveness of routing protocols in wireless ad hoc networks is discussed based on the model of natural average distribution from a practical point of view. In this experiment, a comparison of performance tests of AODV, DSR and TORA is made by OPNET. From simulation results, it can be found that the performances of the three routing protocols are different and different protocols have different superiority in certain environment. These experimental results are provided as a basis for hardware implementation of wireless ad hoc networks.