Air-ratio relates closely to pollution reduction, fuel efficiency and driveability among all of the engine control variables. Maintaining the air-ratio to be the stoichiometric value can obtain the best balance between power output and fuel consumption. The paper presents a nonlinear model predictive control algorithm for air-ratio regulation based on online neural network. The control algorithm has been implemented on a real car to test. The actual test results in experiments show that the proposed control algorithm can regulate the engine air-ratio to the stoichiometric value (1.0) under external disturbance with less than 5% tolerance. © 2010 IEEE.