This research project involved work into the application of a Self Tuning Regulator (STR), one type of adaptive control technique used in the real-time control of internal combustion engines in order to enhance the air/fuel controllability. We concentrated on the target air/fuel and output air/fuel values, and applied adaptive control to the feedback system. With this structure, equivalence transformation of a conventional PID feedback controller to STR can be performed. Through simulation, it was confirmed the effectiveness of the controllability and disturbance robustness, and controllability verification tests were performed using real-time engine control on an actual vehicle in the FTP75 LA-4 mode. This resulted in a dramatic improvement in air/fuel controllability.