Unmanned aerial vehicles (UAVs) have received considerable attention in many civilian and military fields. Performing environmental monitoring, and surveying applications have very specific constraints on power, cost, weight, and flight endurance. Because of the limited flight duration of electric UAVs that are powered by only batteries, this paper proposed the development of a new power system for long endurance UAVs propulsion application. This power system consists of a fuel cell stack, a pack of Lithium-ion batteries, and a DC-DC buck converter. Normally, using Jacobian linearization via a Taylor series expansion at the nominal operating point cannot achieve satisfactory dynamic performance under sudden changes in the loads. Therefore, nonlinear dynamic models of both the fuel cell and the Lithium-ion battery are developed. The structure of buck converter is also studied and its different parts are introduced. A buck converter optimized PID controller is designed to convert the stack voltage into standard DC and to achieve a satisfactory transient behavior. This PID controller is tuned by the gradient descend multi-parameter-optimization technique. The proposed system is modeled and simulated using MATLAB/Simulink software. Simulation results showed that a hybrid combination of hydrogen fuel cells and Lithium-ion batteries can fulfill the requirements of a long endurance UAV power source and verified the capability of the designed system to maintain the output voltage constant under sudden changes in the loads.