The present invention relates to an olefin-based polymer, which has (1) a density (d) ranging from 0.85 to 0.90 g/cc, (2) a melt index (MI, 190° C., 2.16 kg load conditions) ranging from 0.1 g/10 min to 15 g/10 min, (3) a molecular weight distribution (MWD) in a range of 1.0 to 3.0, and (4) a number average molecular weight (Mn) and a Z+1 average molecular weight (Mz+1) satisfying the Equation 1, {Mn/(Mz+1)}×100>15. The olefin-based polymer according to the present invention is a low-density olefin-based polymer and exhibits excellent tensile strength due to having improved molecular weight as compared to the flow index.