The design of a fuel cell stack is important to achieve optimal output power. This study focuses on the evaluation of a fuel cell system for unmanned aerial vehicles (UAVs). Low temperature proton exchange membrane (LTPEM) fuel cells are the most promising energy source for robot applications because of their unique advantages such as high energy density, cold startup, and quick response during operation. In this paper, a 600 W open cathode LTPEM fuel cell was tested to evaluate the performance and to determine optimal operating conditions. The open cathode design reduces the overall size of the system to meet the requirements for robotic applications. The cruise power requirement of 600 W was supported entirely by the fuel cell while the additional power requirements during takeoff was extended using a battery. A peak of power of 900 W is possible for 10 minutes with a lithium polymer (LiPo) battery. The system was evaluated under various load cycles as well as start-stop cycles. The system response from no load to full load meets the robot platform requirements. The total weight of the stack was 2 kg, while the overall system, including the fuel processing system and battery, was 4 kg.