A method of preparing an olefin oligomer by polymerizing an olefin having 6 or more carbon atoms, wherein said olefin is polymerized in the presence of a catalyst consisting of (a) a polyhydric alcohol derivative obtained by replacing the hydrogen atoms of all the hydroxyl groups of the polyhydric alcohol by either acyl groups exclusively or by acyl groups and alkyl groups, both of said groups having 1 - 20 carbon atoms, (b) an aluminum halide in an amount of 1.1 - 2.8 mole per one ester bond or ether bond possessed by said polyhydric alcohol derivative and (c) metallic aluminum powder.