An ethylene copolymer obtained by polymerization of ethylene and a C 3 -C 18 α-olefin comonomer, wherein the ethylene copolymer is prepared by performing a method comprising (a) preparing a first copolymer having a density of 0.860 to 0.940 g/cm 3 and a melt index of 0.001 to 2.0 g/10min; and (b) preparing the ethylene copolymer having a density of 0.910 to 0.960 g/cm 3 and a melt index of 0.5 to 20 g/10min by using the first copolymer from the step (a), wherein the steps (a) and (b) are carried out using a catalyst composition including a transition metal catalyst represented by chemical formula 1 below: in Chemical Formula 1, M is a transition metal of group 4 on the periodic table of elements; C p is a cyclopentadienyl ring which is η 5 -linkable to the core metal M, or a fused ring containing a cyclopentadienyl ring, wherein the cyclopentadienyl ring or the fused ring containing a cyclopentadienyl ring may be further substituted with one or more substituents selected from (C 1 -C 20)alkyl, (C 6 -C 30)aryl, (C 2 -C 20)alkenyl, and (C 6 -C 30)ar(C 1 -C 20)alkyl; R 1 through R 4 each are independently a hydrogen atom, a halogen atom, (C 1 -C 20)alkyl, (C 3 -C 20)cycloalkyl, (C 6 -C 30)aryl, (C 6 -C 30)ar(C 1 -C 10)alkyl, (C 1 -C 20)alkoxy, (C 3 -C 20)alkylsiloxy, (C 6 -C 30)arylsiloxy, (C 1 -C 20)alkylamino, (C 6 -C 30)arylamino, (C 1 -C 20)alkylthio, (C 6 -C 30)arylthio or nitro, or R 1 through R 4 each may be linked to an adjacent substituent via (C 3 -C 12)alkylene or (C 3 -C 12) alkenylene with or without containing a fused ring, to form an alicyclic ring, or a monocyclic or polycyclic aromatic ring; Ar 1 is (C 6 -C 30)aryl or (C 3 -C 30)heteroaryl containing one or more selected from N, O and S; X 1 and X 2 each are independently a halogen atom, (C 1 -C 20)alkyl, (C 3 -C 20)cycloalkyl, (C 6 -C 30)ar(C 1 -C 20)alkyl, (C 1 -C 20)alkoxy, (C 3 -C 20)alkylsiloxy, (C 6 -C 30)arylsiloxy, (C 1 -C 20)alkylamino, (C 6 -C 30)arylamino, (C 1 -C 20)alkylthio, (C 6 -C 30)arylthio, or R 11 through R 15 each are independently a hydrogen atom, a halogen atom, (C 1 -C 20)alkyl, (C 3 -C 20)cycloalkyl, (C 6 -C 30)aryl, (C 6 -C 30)ar(C 1 -C 10)alkyl, (C 1 -C 20)alkoxy, (C 3 -C 20)alkylsiloxy, (C 6 -C 30)arylsiloxy, (C 1 -C 20)alkylamino, (C 6 -C 30)arylamino, (C 1 -C 20)alkylthio, (C 6 -C 30)arylthio or nitro, or R 11 through R 15 each may be linked to an adjacent substituent via (C 3 -C 12)alkylene or (C 3 -C 12) alkenylene with or without containing a fused ring to form an alicyclic ring, or a monocyclic or polycyclic aromatic ring; and alkyl, aryl, cycloalkyl, aralkyl, alkoxy, alkylsiloxy, arylsiloxy, alkylamino, arylamino, alkylthio or arylthio of R 1 through R 4, R 11 through R 15, and X 1 and X 2; the ring formed by linkage of R 1 through R 4 each or R 11 through R 15 each to an adjacent substituent via alkylene or alkenylene; and aryl or heteroaryl of Ar 1 may be further substituted with one or more substituents selected from a halogen atom, (C 1 -C 20)alkyl, (C 3 -C 20)cycloalkyl, (C 6 -C 30)aryl, (C 6 -C 30)ar(C 1 -C 10)alkyl, (C 1 -C 20)alkoxy, (C 3 -C 20)alkylsiloxy, (C 6 -C 30)arylsiloxy, (C 1 -C 20)alkylamino, (C 6 -C 30)arylamino, (C 1 -C 20)alkylthio, (C 6 -C 30)arylthio, nitro and hydroxyl, wherein the ethylene copolymer has an activation energy of 40 kJ/mol or more and a molecular weight distribution (Mw/Mn) of 2.4 or more, and satisfies equations 1 and 2 below at a section where an oscillatory torque is 60 to 6,000 μN.m: P ≤ 168.8 − 97.4 × t + 32.2 × t 2 − 4.0 × t 3 P ≥ 204.3 − 157.7 × t + 58.0 × t 2 − 7.5 × t 3 wherein the equations 1 and 2 above, t is an oscillatory torque (μN.m)and P is a phase angle (°), and wherein a content of C 3 -C 18 α-olefin comonomer is 1 to 40% by weight.