A method for forming a photo-mask is provided. A first photo-mask pattern relating to a first line, an original second photo-mask pattern relating to a first via plug, and a third photo-mask pattern relating to a second line are provided. A first optical proximity correction (OPC) process is performed. A second OPC process is performed, comprising enlarging a width of the second photo-mask pattern along the first direction to form a revised second photo-resist pattern. A contour simulation process is performed to make sure the revised second photo-mask pattern is larger or equal to the original second-mask pattern. The first photo-mask pattern, the revised second photo-mask pattern, and the third photo-mask pattern are output. The present invention further provides an OPC method.