In a photosensitive resin, a photoresist composition including the photosensitive resin and a method of forming a photoresist pattern using the photoresist composition, the photosensitive resin includes a hydrophobic terminal group having at least five carbon atoms and a blocking group. The photosensitive resin has a weight average molecular weight of from about 6,000 up to about 9,500. The photoresist composition including the photosensitive resin may form a photoresist pattern having a reduced line edge roughness and a fine line width with accuracy.