Photoresist polymers and photoresist compositions containing the same. Photoresist patterns of less than 50 nm are achieved with EUV (Extreme Ultraviolet) as an exposure light source with photoresist compositions comprising (i) a photoresist polymer comprising a polymerization repeating unit of Formula 2 or (ii) a photoresist polymer comprising a polymerization repeating unit of Formula 3 with polyvinylphenol. As a result, excellent etching resistance can be secured although the photoresist patterns have a very small thickness.wherein R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12, a, b, c, d, e, f and g are as defined in the specification.