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: M.sup.1R.sup.1.sub.1R.sup.2.sub.mR.sup.3.sub.n or R.sup.2.sub.mR.sup.3.sub.nM.sup.1R.sup.1.sub.1M.sup.1R.sup.2.sub.mR.sup.3- .sub.n [Formula 1 ]in Formula 1, M.sup.1 is selected from the group consisting of Group 2A, 2B and 3A of the Periodic Table, R.sup.1 is cyclic hydrocarbyl group of 5 to 30 carbon atoms, R.sup.2 and R.sup.3 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 M.sup.1, Q is a divalent group; M.sup.2R.sup.4.sub.pX.sub.q [Formula 2 ]in Formula 2, M.sup.2 is Ti, Zr or Hf; R.sup.4 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 M.sup.2.