The dynamic topology of a mobile ad hoc network (MANET) poses a real challenge in the design of a MANET routing protocol. Over the last 10 years, a variety of routing protocols have been developed and their performance simulations are made by network researchers. Most of the previous research on MANET routing protocols have focused on simulation study by varying network parameters, such as network size (node density), pause times, or node mobility independently. This paper considers the problem from a different perspective, using a simulation model the combined effect of node density and packet length; node density and mobility on the performance of a typical 802.11 MANET is investigated. This is a common and realistic scenario in MANETs where nodes move around, join and leave the network at any time. Based on the QoS (end-to-end delay, throughput), routing load and packet retransmissions, this paper systematically analyzes the performance of four diverse MANET routing protocols with the different simulation model and configurations, and drew more complete conclusions.