A method for producing a stereoregular polyolefin in the presence of a catalyst comprising a transition metal compound and an organometallic compound, which comprises polymerizing at least one olefin in the presence of a catalyst system comprising:(A) a solid catalyst component comprising Mg, Ti, halogen and an electron donative compound,(B) at least one member selected from the group consisting of organometallic compounds of metals of Groups IA, IIA, IIB, IIIB and IVB of the Periodic Table,(C) an electron donative compound, and(D) a boron compound of the formula R.sup.1.sub.n BX.sub.3-n wherein R.sup.1 is a hydrocarbon group and/or a halogenated hydrocarbon group, X is a halogen atom, and 0.ltoreq.n.ltoreq.3.