An acid-amplifier having an acetal group and a photoresist composition including the same, are disclosed. The acid-amplifier produces an acid (second acid) during a post-exposure-bake (PEB), which is induced by an acid (first acid) generated from a photo-acid generator (PAG) at the exposure process so that a line edge roughness (LER) of the photoresist pattern and photoresist energy sensitivity are improved. The acid-amplifier has a structure of following Formula 1. in Formula 1, R is C4˜C20 mono-cyclic or multi-cyclic saturated hydrocarbon, R1 is C1˜C10 linear hydrocarbon, C1˜C10 perfluoro compound or C5˜C20 aromatic compound, Ra and Rb are independently hydrogen atom or C1˜C4 saturated hydrocarbon and A is independently oxygen atom (O) or sulfur atom (S).