Abstarct: In this thesis, two methods for optimal sizing of power sources in plug-in fuel cell electric vehicle have been introduced. At first, a theoretical model is presented to describe the relationship between components’ parameters and vehicle performance. It is understood that when the electric motor works in a normal mode, the maximum velocity and driving distance are influenced linearly by the parameters of the components, e.g. fuel cell efficiency, fuel cell output power, stored hydrogen mass, vehicle auxiliary power, battery capacity, and battery average resistance. Next, It is proposed an optimal sizing method for hybrid powertrains that minimize fixed and operation costs of plug-in fuel cell electric vehicle. Where fixed cost is the fixed cost of the fuel cell system and battery packages and operation cost is the lifetime operation cost of the two power sources (hydrogen and electricity). By solving the optimization problem and obtain the optimal values, it is confirmed the performance accuracy of vehicle by using ADVISOR software by implementing in two standard cycles.