An air-fuel ratio control method for an internal combustion engine. The air-fuel ratio of an air-fuel mixture supplied to the engine is feedback-controlled to a desired air-fuel ratio depending on operating conditions of the engine in response to output from an exhaust gas ingredient concentration sensor. The desired air-fuel ratio is settable to a value leaner than a stoichiometric air-fuel ratio when the temperature of the engine is above a predetermined reference value. The method comprises the steps of detecting a reduction ratio to which the transmission has been set, and changing the predetermined reference value of the temperature of the engine depending on the detected reduction ratio. The predetermined reference value of the temperature of the engine is set to a lower value as the reduction ratio of the transmission is smaller.