The present invention relates to an olefin-based copolymer having novel crystalline properties and capable of providing a molded article with further improved strength and impact strength when compounded with other resins, and a method for preparing the same. The olefin-based copolymer is an olefin-based copolymer comprising an ethylene repeating unit and an α-olefinic repeating unit. The olefin-based copolymer comprises polymer fractions defined by three different peaks at a predetermined temperature when analyzed by cross-fractionation chromatography.