Provided are a photoresist composition and a method of manufacturing a semiconductor device using the same. The method of manufacturing a semiconductor device comprises forming a mask layer and a photoresist layer on a substrate, forming a photoresist pattern by patterning the photoresist layer, forming a mask pattern by patterning the mask layer through the photoresist pattern and forming a pattern by etching the substrate using the mask pattern, wherein the formation of the photoresist layer comprises forming the photoresist layer, using a photoresist composition comprising a polymer which includes a protecting group that causes decarboxylation by radical.