            !! 1          Fuel cells are electrochemical devices that convert the chemical energy of a reaction directly into electrical energy. Among the several types of fuel cell, Proton Exchange Membrane Fuel Cell (PEMFC) is a suitable choice for distributed energy sources. In this paper, a mathematical model of 750W PEMFC is developed. This model describes the behaviour of PEMFC under steady(state and transient conditions. Novel feature of this model is integration of all possible dynamic equations like dynamics of the charge equations like dynamics of the charge double layer capacitance, lumped fuel cell body dynamics and anode and cathode channel dynamics into a single model. The VI characteristic of PEMFC is obtained for different values of input parameters. The transient response of the PEMFC model over short and long(time periods is analyzed. Finally, the behaviour of the PEM fuel cell model under a resistive load is evaluated.  PEMFC, Transient, Charge double layer   Fuel cell is a high quality green energy source and it is gaining much attention because of its light weight, compact size, low maintenance, high efficiency and reliability. It serves as a potential source for electric power generation for standalone as well as for grid(tied applications. Compared with the other types of fuel cells, PEMFC shows promising results with its advantages such as low temperature, high power density, fast response, and zero emission if it is run with pure hydrogen, and it is suitable for use in portable power supply, vehicles, and residential and distributed power plants. In this paper, a mathematical model is proposed to simulate a PEMFC which accounts for the effects of different dynamic conditions in load current, pressure of input reactant gases, and fuel cell operating temperature. The PEMFC model is simulated in MATLAB. The polarization curves (V(I characteristics of the PEMFC) are obtained for different values of input variables. The transient response of the PEMFC model over short and long(time periods is analyzed. Section II gives the basic operation of PEMFC. Section III presents the modeling aspects of PEMFC. Section IV,V and VI presents the steady(state and transient response of PEMFC. SectionVII discusses about the polarization curves of PEMFC.      PEMFC primarily consists of three components: a negatively charged electrode (cathode), a positively charged electrode (anode) and a solid polymer electrolyte membrane. Hydrated hydrogen gas is supplied at the anode and air is supplied at the cathode. At the anode, hydrogen gas in the presence of the platinum catalyst is ionized into positively charged hydrogen ions and negatively charged electrons. The reaction at the anode is: ( 2 2 2  " " + = + (1) The polymer membrane permits only positively charged hydrogen ions to flow from the anode to the cathode as shown in Figure 2.1.   !"# $  The PEMFC have a robust design and are relatively easy to build. A single fuel cell produces an open(circuit voltage of 0.7(1V. Several fuel cells are electrically connected in series forming a stack which provides a fairly large power at higher voltage and current levels. % &’   Different models of PEM fuel cell are reported in the literature [2].This paper focuses on static and dynamic model of fuel cell system intended for power generation application (standalone/grid connected operation). The model is based on electrochemical equations that take into consideration the main operational parameters of fuel cell such as the operational electrical current, temperature and the transportation of gases. It depicts how the output voltage and power of fuel cell is affected due to changes in the load. The proposed model is simulated using MATLAB /Simulink and it is used to analyze the transient behaviour of the fuel cell. The VI characteristics of the PEM fuel cell are obtained for different values of input variables. An open circuit output voltage of the PEM fuel cell, voltage losses, formation of charge double layer in the PEM fuel cell, along with a mass balance and thermodynamic energy balance in the fuel cell system are modeled. Finally, the dynamic response of the fuel cell model over short and long time periods is analyzed.              !" ##! $%&’ (% )*+&           !" ##! $%&’ (%