A method optical proximity correction includes the following steps. First, a layout of an integrated circuit with an exposure intensity specification is provided. The integrated circuit includes a plurality of patterns and each pattern has an exposure intensity distribution. Second, a quadratic polynomial equation of each exposure intensity distribution is approximated. Third, a local extreme intensity of each exposure intensity distribution is computed by fitting the quadratic polynomial equation. Fourth, the local extreme intensity is determined whether violating the exposure intensity specification or not. Then, the layout is corrected when the local extreme intensity violates the exposure intensity specification.