An olefin polymerization catalyst which includes an organometallic compound of the following Formula 1; aluminoxane; and an organic transition metal compound of the following Formula 2: M1R11R2mR3n or R2mR3nM1R11M1R2mR3n  [Formula 1 ] in Formula 1, M1 is selected from the group consisting of Group 2A, 2B and 3A of the Periodic Table, R1 is cyclic hydrocarbyl group of 5 to 30 carbon atoms, R2 and R3 are independently hydrocarbyl group of 1 to 24 carbon atoms, l is an integer of more than 1, m and n are independently an integer of 0 to 2, l+m+n is equal to the valence of M1, Q is a divalent group; M2R4pXq  [Formula 2 ] in Formula 2, M2 is Ti, Zr or Hf; R4 is cyclic hydrocarbyl group of 5 to 30 carbon atoms, X is halogen atom, p is an integer of 0 or 1, q is an integer of 3 or 4, p+q is equal to the valence of metal M2.