An air-fuel ratio control system maintaining constant an oxygen storage amount or oxygen release amount per unit time with respect to an exhaust purification catalyst having an oxygen storage capacity even if the intake air amount changes is provided.An air-fuel ratio control system of an internal combustion engine having an intake air amount detecting means, a linear air-fuel ratio sensor arranged at an upstream side of an exhaust purification catalyst, an O2 sensor arranged at a downstream side of said exhaust purification catalyst, a target air-fuel ratio controlling means for performing feedback control of a target air-fuel ratio of exhaust flowing into the exhaust purification catalyst based on output information from the intake air amount detecting means and the O2 sensor, and a fuel injection amount controlling means for performing feedback control of the fuel injection amount based on output information of the linear air-fuel ratio sensor so as to achieve the target air-fuel ratio, characterized in that the target air-fuel ratio controlling means performs feedback control of the target air-fuel ratio so that even when the intake air amount changes, a correction amount per unit time of an oxygen storage amount of the exhaust purification catalyst is made constant.