The present description relates to an olefin block copolymer having excellences in elasticity, heat resistance, and processability. The olefin block copolymer includes a plurality of blocks or segments, each of which includes an ethylene or propylene repeating unit and an α-olefin repeating unit at different weight fractions. In the olefin block copolymer, a first derivative of the number Y of short-chain branches (SCBs) per 1,000 carbon atoms of each polymer chain contained in the block copolymer with respect to the molecular weight X of the polymer chains is a negative or positive number of −1.5×10−4 or greater; and the first derivative is from −1.0×10−4 to 1.0×10−4 in the region corresponding to the median of the molecular weight X or above.