A method for treating an olefin polymer, which comprises contacting an olefin polymer obtained by polymerizing an olefin in the presence of a catalyst composed of (A) a highly active transition metal compound, (B) an organoaluminum compound and (C) a carbonyl- or Si-containing electron donor, with (D) an organometallic compound of a metal of Groups I to III of the periodic table after the polymerization. This method can markedly reduce the amount of an electron donor remaining in an olefin polymer after polymerization, the electron donor being an ingredient of a catalyst used in the polymerization and constituting the main cause of an offensive odor which the resulting polymer gives off.