In a photosensitive polymer, a photoresist composition including the photosensitive polymer and a method of forming a photoresist pattern using the photoresist composition, the photosensitive polymer has a weight average molecular weight of from about 1,000 up to about 100,000 and a repeating unit represented by the following chemical formula (1). wherein R.sub.1 represents hydrogen or an alkyl group having 1 to 10 carbon atoms, R.sub.2 represents an acid-labile hydrocarbon group having 3 to 12 carbon atoms, and n represents an integer greater than or equal to 1. The photoresist composition having good reproducibility and stability may form a photoresist film having a substantially uniform thickness, and may form a fine pattern with accuracy.