An optimum humidification of the reactant gases of proton exchange membrane (PEM) fuel cell extremely affects its performance. Here, an analytic model of a membrane humidifier for PEM fuel cell is proposed where the effect of mass flow rates, inlet temperatures and pressures are investigated. The governing equations: water transfer equation and the law of conservation of energy in whole humidifier are written, which form a Non-linear system of equations, solved through FORTRAN software. At each stage, the outlet temperatures, the water transfer rate, relative humidity and the dew point at dry side outlet are calculated and discussed. The closer the dry side outlet dew point to the wet side inlet dew point, leads to the better humidifier performance. The results show that an increase in mass flow rate at dry side inlet leads to the weaker humidifier performance; while, an increase in mass flow rate at wet side inlet leads to the better performance. An increase in the pressure at dry side inlet enhances humidifier performance; while, the pressure at wet side inlet does not affect significantly on humidifier performance. Here, preheating the dry gas is not essential and use the cooler wet gas is recommended