A catalyst composition and a method for preparing an olefin polymer by using the same are provided herein. In some embodiments, a catalyst composition includes a transition metal compound having a compound (A) represented by Chemical Formula 1 and a compound (B) represented by Chemical Formula 2 present in a molar ratio of 1.6:1 to 18. The catalyst composition has high activity in an olefin polymerization reaction and can contribute to a reduction of catalyst cost. The catalyst composition aides in high copolymerizability of olefin monomers, and the produced olefin polymers can exhibit excellent processability and long-term physical properties, suitable for a pipe.