The present invention provides a cross-linker monomer of formula 1, a photoresist polymer derived from a monomer comprising the same, and a photoresist composition comprising the photoresist polymer. The cross-linking unit of the photoresist polymer can be hydrolyzed (or degraded or broken) by an acid generated from a photoacid generator on the exposed region. It is believed that this acid degradation of the cross-linking unit increases the contrast ratio between the exposed region and the unexposed region. The photoresist composition of the present invention has improved pattern profile, enhanced adhesiveness, excellent resolution, sensitivity, durability and reproducibility. where A, B, R1, R2, R3, R4, R5, R6 and k are as defined herein.