The present invention relates to an elastic terpolymer that can acquire good mechanical properties and elasticity (flexibility) at the same time, and a preparation method thereof.The elastic terpolymer, which is a copolymer of 40 to 70 wt % of ethylene, 15 to 55 wt % of a C3-C20 alpha-olefin, and 0.5 to 20 wt % of a diene as obtained in the presence of a group 4 transition metal catalyst, satisfies that: (i) the weight average molecular weight measured by GPC is 100,000 to 500,000; and (ii) the ethylene content x (wt %) and the crystallization enthalpy y (ΔHcJ/g) of the copolymer measured by DSC satisfy a defined relational expression.