From the simulation study, it is concluded that a fully humidified anode and a dry cathode aretypically required to maximize the power density delivered by a PEM fuel cell, unless a high air flow rate is employed, in which case the cathode should be operated with an intermediate relative humidity. Based on the developed understanding of the individual influences of inlet anode and cathode humidification, two examples on the use of inlet relative humidity control for maximizing the volumeric power density and operating range of PEM fuel cell have been discussed. Its usefulness as a simple and effective method for the performance optimization of PEM fuel cell has been theoretically demonstrated.