An optical proximity correction (OPC) method for correcting a photomask layout. The photomask layout has at least a photomask pattern. The steps of the OPC method include collecting an assist feature bias of a predetermined assist feature, performing a rule-based OPC by taking account of the assist feature bias to compute a target bias of the photomask layout, outputting a corrected photomask layout according to the target bias, and adding the predetermined assist feature to the corrected photomask layout.