The present description relates to an olefin block copolymer having excellences in elasticity and heat resistance and its preparation method. 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. The olefin block copolymer has a density of 0.85 to 0.92 g/cm3, and density X (g/cm3) and TMA (Thermal Mechanical Analysis) value Y (° C.) satisfy a defined relationship.