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, a stabilization fuel quantity increasing factor 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. Air-fuel ratio feedback control is started when the air-fuel ratio corresponds to a stoichiometric air-fuel ratio. After starting air-fuel ratio feedback control, an unburned fuel quantity compensating value is set based on the stabilization fuel quantity increasing factor in effect at that point in time and added to the target air-fuel ratio revising coefficient while, simultaneously, the stabilization fuel quantity increasing factor is set to zero.