(57) The new metallocene catalysts according to the present invention are prepared by reacting a metallocene compound with a compound having at least two functional groups. The metallocene compound is a transition metal compound, which is coordinated with a main ligand such as cyclopentadienyl group, and an ancillary ligand. The functional groups in the compound are selected from the group consisting of a hydroxyl group, an alkyl or aryl magnesium halide, a thiol group, a primary amine group, a secondary amine group, a primary phosphorous group, a secondary phosphorous group, etc. The metallocene catalysts according to the present invention have a structure in which an ancillary ligand of a metallocene compound is bonded to the functional groups of a compound having at least two functional groups. A structure of the metallocene catalysts can be varied with the type of a metallocene compound and a compound having at least two functional groups, and the molar ratio of each reactant. The metallocene catalysts are employed with a co-catalyst for olefin polymerization. The co-catalyst is an organometallic compound, or a mixture of non-coordinated Lewis acid and alkyl aluminium as it is well known in the art.