In MPLS networks, the higher-priority LSP (label switching path) will preempt the resource of lower-priority LSP when its bandwidth resource is limited. At the time, the lower-priority LSP is destroyed, and Its bandwidth resource is released to the higher-priority LSP. The destroyed path has to be rerouted by selecting another LSP; the new LSP maybe quickly suffer another bandwidth resource preemption again. If this scenario occurs frequently, routers would be subjected to the superfluous overload, and the quality of delivering flow can not be guaranteed. In a previous Chin et al. (see IASTED CCN 2002) had proposed a new policy to avoid the preemption because of flow priority and load balancing in the MPLS networks. In this paper, the policy supports the end-to-end QoS in DiffServ-aware MPLS networks is discussed. Moreover, many simulations are done to compare the QoS of all service levels in DiffServ which constraint-based routed (CR) scheme includes the new policy of Chin et al. The simulation results indicate that adding our policy to CR is better then the traditional CR.