This paper presents an optimal design methodology of fuel cell hybrid power source under energy constraints for transport applications. A parallel architecture of hybrid energy conversion composed of a fuel cell and a battery pack is considered. The proposed methodology is based on optimization approach using Particle Swarm Optimization technique, associated to an Optimal Control algorithm used as cost function to take into account the energy behavior. The objective of this study is to optimize the various factors affecting system design to determine the best parameters meeting design requirements, which are fuel saving in this first study.Simulation results are presented to validate the relevance of the proposed optimal design and to demonstrate the performances of the proposed methodology according to a real requirements.