A method of preparing a polyolefin oil in the present invention comprises polymerizing an olefin having 6 or more carbon atoms in the presence of a mixture of a polyhydric alcohol derivative and an aluminum halide, said derivative being obtained by replacing hydrogen atoms of all the hydroxyl groups of the polyhydric alcohol by alkyl groups having 1-20 carbon atoms. The thus prepared polyolefin oil scarcely contains halogen, and it has a wide range of use such as, for instance, a lubricant, a cosmetic base, a textile finishing agent, etc.