In an air/fuel ratio control system for an internal combustion engine, including load detection means for detecting a load of said engine using a plurality of operating parameters, and air/fuel ratio control means for controlling an air/fuel ratio of mixture to be supplied to the engine toward a reference value as a target when an engine load detected by said load detection means is within a first load range, and for controlling the air/fuel ratio toward a value larger than said reference value when said engine load detected by said load detection means is within a second load range which is on lighter side of said first load range, the improvement wherein a varying speed detection means is provided for detecting a speed of variation of at least one of said operating parameters, and said air/fuel ratio control means is operated to control the air/fuel ratio toward the reference value while the engine load is within the second load range if the speed of variation of at least one of the operating parameters detected by the varying speed detection means exceeds a predetermined value.