Provided is a method of fabricating a semiconductor device using a curvilinear OPC method. The method of fabricating the semiconductor device includes performing an optical proximity correction (OPC) step on a layout to generate a correction pattern, the correction pattern having a curvilinear shape, performing a mask rule check (MRC) step on the correction pattern to generate mask data, and forming a photoresist pattern on a substrate using a photomask, which is manufactured based on the mask data. The MRC step includes generating a width skeleton in the correction pattern, generating a width contour, which satisfies a specification of a mask rule for a linewidth, from the width skeleton, and adding the correction pattern and the width contour to generate an adjusting pattern.