A method of preparing an olefin oligomer by polymerizing an olefin having more than 6 carbon atoms, wherein said olefin is to be polymerized in the presence of a catalyst consisting of (a) polyhydric alcohol derivatives obtained by substituting hydrogen atoms of all the whole hydroxyl groups of polyhydric alcohol by either acyl group exclusively or acyl group and alkyl groups, both said acyl group and alkyl group having 1 - 20 carbon atoms, (b) aluminum halide in an amount of 0.7 - 1.0 mole per each ester bond and ether bond possessed by said polyhydric alcohol derivative and (c) metallic aluminum powder.