The present invention relates to a method of polymerization or co-polymerization of olefin under the catalyst system which comprises (a) a solid complex titanium catalyst comprising of a magnesium compound, a titanium compound, and an internal electron donor; (b) an organometallic compound of metal of Group I or Group III on the Periodic Table; and (c) two of external electron donors comprising an external electron donor (1) and an external electron donor (2), wherein the ratio of the MFR (1) to the MFR (2) is 2˜10, the MFR (1) being that of the homopolymer (1) obtained by using said external electron donor (1) with said solid complex titanium catalyst (a) and the organometallic compound (b), and the MFR (2) being that of the homopolymer (2) obtained by using said external electron donor (2) under the identical polymerization reaction conditions of said external electron donor (1), and wherein the NMR pentads of the homopolymer (1) and the homopolymer (2), respectively, are 95.5% or more.