In a photosensitive resin, a photoresist composition having the photosensitive resin, and a method of forming a photoresist pattern by using the photoresist, the photosensitive resin includes a blocking group substituted for an acid. The photosensitive resin has a weight-average molecular weight of from about 6,000 up to about 8,000. The photosensitive resin has a blocking ratio of from about 5% up to about 40%.