An engine air-fuel ratio control system is configured to use a rich air-fuel ratio immediately after starting an engine such that the air-fuel ratio converge rapidly toward a stoichiometric value and then afterwards start an air-fuel ratio feedback control. Upon determining an air-fuel ratio sensor is active based on the output of the air-fuel ratio sensor and the amount of time elapsed since an engine was started, a stabilization fuel quantity increasing value that is a component of a target air-fuel ratio revising coefficient is decreased at a higher rate than the rate used before the air-fuel ratio sensor was determined to be active. During the same period, an air-fuel ratio feedback revising coefficient is held at a reference value. After the output of the air-fuel ratio sensor reaches a value corresponding to a stoichiometric air-fuel ratio, an air-fuel ratio feedback control is started.