To provide a polymer for a photoresist having high transmittance, etching resistance, thermal resistance, adhesiveness and low light absorbance, and high affinity for a developer in order to obtain a pattern improved of LFR (line edge roughness), and to provide a photoresist composition including the polymer, and a method for forming a pattern by using the composition.

The invention relates to the photoresist polymer and the photoresist composition, more in detail, a compound expressed by formula (1) (wherein R is CH2, CHCH2 or C(CH2)2; R' and R" are each H or CH3; R1, R2, R3, R4 and R5 are each H, F, a 1-20C alkyl, a 1-20C perfluoroalkyl, a 1-20C alkyl containing an ether group (-O-), etc.; m is an integer of 0-30) used as the photoresist monomer, and the photoresist polymer comprising the monomer, and the photoresist composition comprising the polymer. The photoresist composition has high transmittance at wavelengths of 193 nm and 157 nm, etching resistance, thermal resistance, adhesiveness and low light absorbance, and high affinity to a developer, thereby can improve the LFR.