This paper deals with the hybridisation of a small PEM (Proton Exchange Membrane) fuel cell system by an ultracapacitors bank. This kind of hybrid source could be used for instance to supply the DC brushless machines of a small electric vehicle. Modelling and simulation are derived from ADVISOR software. Two energy flow control strategies are described; globally, the fuel cell should operate in a power range, which ensures a good efficiency for the whole fuel cell system and, at the same time, an ultracapacitor state of charge about 75%.