An integration-type owmeter, composed of an ultrasonic owmeter and an integration circuit, is used to measure the air mass for transient air-fuel ratio (AFR) control of a port fuel injection (PFI ) spark ignition engine. Also, the air mass and required fuel mass in the cylinder are accurately calculated for precise AFR control. The proposed method can signi cantly improve the accuracy of measuring air mass inducted through a throttle valve. Air mass passing into a cylinder is estimated using the measured air mass at the throttle valve and intake manifold pressure. A simple two-constant fuel model is used for a dynamic fuel model. Control parameters from the air and fuel dynamics are applied to minimize AFR excursions, and a four-cylinder, 1.5 l PFI engine was used to demonstrate this AFR control strategy. Results show that the AFR followed a command value with a peak error of 4 per cent during throttle transients at various operating points.