Polymer Electrolyte Fuel Cells (PEFCs) operate in wide-range changes in temperature, humidity, and electric current for automotive applications. In order to operate automotive PEFC efficiently, optimal air supply is required to adjust to these changes. This paper presents an air-supply optimization process that consists of experiments, modeling of the PEFC system, and optimization. The objective is to establish an air supply suitable for the required power for PEFC system and optimized with a Lagrange multiplier. Our simplified PEFC system model is used as a constraint for optimization problem. The result of this paper presents that efficient operation of PEFC system can be achieved by air-supply optimization.