A fuel cell stack (FCS) converts the chemical energy to be the electrical energy through an electrochemical process. One type of FCS is proton exchange membrane (PEM) with hydrogen and oxygen as the main chemical energy source. The PEMFCS has been applied widely for operating direct current (DC) loads. However, the optimum sizing of FCS, DC load and the other supported devices are not considered well, thus the system performance does not have function optimum fully. This paper present the FCS integrated by boosted DC-DC converter for running DC load. The optimum sizing of FCS and boosted DC-DC converter are explained to obtain the required number of FCS and boosted DC-DC converter. A simulation of the FCS integrated boosted DC-DC converter is conducted using SIMULINK MATLAB. The simulation results show that the FCS and boosted DC-DC converter have good performance in the operation of DC load.