A method of manufacturing a polyolefin using a catalyst system of an ingredient (A) and ingredient (B), with ingredient A being obtained through successive reaction steps. Ingredient A is prepared by forming a mixture of (1) the combination of (a) metallic magnesium or an oxygen-containing organic compound of magnesium with (b) a mixture of a monohydroxylated organic compound and a polyhydroxylated organic compound, and (2) oxygen-containing organic compounds of titanium. The resulting solution is reacted in sequence with (3) at least one first halogenated aluminum compound, (4) at least one silicon compound and (5) at least one second halogenated aluminum compound. Compounds 3 and 5 are added at temperatures not exceeding about 25.degree. C. Ingredient B is an organometallic compound of metals from Group Ia, IIa, IIIa or IVa of the Periodic Table. The resulting free-flowing polyolefin is characterized by a large average particle size and small amount of fine particles.