The development of portable computing devices has raised the demand for Quality of Service in MANETs recently. The main objective of the routing protocol in these highly on demand services is to enable successful data packet transportation from one point to another. The collaboration of each network node is essential for the successful transfer of data packets. One of the most popular routing protocols is Dynamic Source Routing (DSR); however selfish nodes drop significant number of packets in Mobile Ad-hoc Network switch affects the quality of services. To solve this problem we present a trust-based priority adaptive QOS optimization technique for DSR in MANET (TP-DSR) to improve the quality of service on the DSR protocol by isolating selfish nodes that either purposefully drop packets or do not reply to route requests in the network. Routing agent collects route information during route discovery and neighbor information from periodic signals send to neighbor nodes. Trust agent calculates the trust value of nodes based on their response time for TRREQ. According to their trust rating, nodes and routes are preserved and updated in order of priority as HP (high priority), MP (medium priority), and LP (low priority) on the cache table. Performance analysis of the system is performed and compared against the original DSR protocol and recent Trust based routing algorithms using Network Simulator 2. The simulation result shows that our proposed system improves the Packet Delivery Ratio to 96.7%, throughput to 103.7Kbytes/sec, end-to-end delay to 0.69 seconds, and routing overhead to 1.89%. Therefore, based on the performance evaluation metrics our scheme is feasible and effective. Key Words: Mobile Ad hoc Network, Priority, Trust, Quality of Service, Selfish Node