This paper presents a power-aware on-demand routing protocol called PAOD, which aims to maximize the system lifetime of MANET. In PAOD, the source is able to anticipate the traffic of the request, and each node on the route makes energy reservation according to the anticipated traffic. Since a node knows not only its physical residual energy, but also the expected energy possibly to be consumed in the future, it will report its energy status more accurately. A new cost function is used in route selection, which takes both shortest-hop and maximum-lifetime into consideration. Some filter mechanisms are proposed based on an energy threshold function, which can remarkably reduce the overhead costs, and improve the performance of PAOD. A novel metric & the standard deviation of node energy is used in the simulation experiments to evaluate the balance of energy consumption in the candidate protocols. Simulation results show that, the energy consumption in PAOD is more balanced than in the common on-demand protocols, and PAOD is with much longer system lifetime, along with higher delivery rate than the common ones.