NOVELTY - A fuel cell system comprises a stack containing unit cells for generating electrical energy through electrochemical reaction of a fuel and an oxidizing agent, a blower configured to recirculate gas exhausted from the stack so as to supply the gas back to the stack, an ejector configured to recirculate the gas exhausted from the stack, receive hydrogen so as to mix hydrogen with the recirculated gas, and supply the mixture to the stack, and a sensor module configured to detect driving condition of a vehicle.    USE - A fuel cell system for vehicles (claimed).    ADVANTAGE - Prediction of recirculated hydrogen amount is highly reliable, a constant amount of stack output is maintained, and performance of a fuel cell system is improved.    DETAILED DESCRIPTION - A fuel cell system comprises a stack containing unit cells for generating electrical energy through electrochemical reaction of a fuel and an oxidizing agent, a blower configured to recirculate gas exhausted from the stack so as to supply the gas back to the stack, an ejector configured to recirculate the gas exhausted from the stack, receive hydrogen so as to mix hydrogen with the recirculated gas, and supply the mixture to the stack, a sensor module configured to detect driving condition of a vehicle, and a control portion configured to control operations of the blower and the ejector based on the driving condition of the vehicle and performance maps of the blower and the ejector. An INDEPENDENT CLAIM is included for a method for controlling a fuel cell system for vehicles which involves detecting driving condition of a vehicle, calculating recirculated hydrogen amount by using the driving condition of the vehicle and performance maps of the blower and the ejector, calculating a stoichiometric ratio (SR) of hydrogen in the stack, comparing the SR of hydrogen in the stack with a predetermined value, and increasing recirculated hydrogen amount if the SR of hydrogen in the stack is smaller than the predetermined value.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart of the method for controlling a fuel cell system for vehicles.