Provided is catalyst composition including a transition metal complex precatalyst represented by Formula 1; a first cocatalyst represented by Formula 2 which is an alkylaluminum compound; and a second cocatalyst represented by Formula 3 which is a salt compound comprising a Bronsted acid cation and a noncoordinating, compatible anion. Here, R1, R2, R3, R4, E, Q1, Q2 and M are defined in the specification. Al(R6)3 Formula 2 Here, R6 is defined in the specification. [L-H]+[ZA4]− Here, L, [L-H]+, Z and A are defined in the specification.A catalyst composition including binuclear transition metal complexes, an alkylaluminum compound, and a salt compound including a Bronsted acid cation, and a noncoordinating, compatible anion, and a method of preparing the catalyst composition are provided. The activity of the catalyst composition has been improved. The catalyst composition can be effectively used for copolymerization of ethylene with monomers having structural inherence, and a polyolefin copolymer having a very low density less than 0.910 g/cc can be obtained by using the catalyst composition.