An air-fuel ratio control system of an internal combustion engine includes a controller that performs air-fuel ratio feedback control through main feedback control and sub feedback control based on the concentrations of oxygen in exhaust gas upstream and downstream of a catalyst respectively, and the controller stores a constant component of a sub feedback correction value of the sub feedback control as a sub feedback learned value, and sets a feedback target value of the main feedback control to values that are different depending on whether or not exhaust gas is recirculated into intake air. When the constant component of the sub feedback correction value obtained when the amount of recirculated exhaust gas is short of its required amount is stored as the sub feedback learned value, the controller corrects the feedback target value of the main feedback control when the recirculation of exhaust gas is not conducted.