A polyolefin superior in particle properties, for example, having an extremely high bulk density, a large average particle diameter and a narrow particle size distribution, and narrow in molecular weight distribution, is obtained in high yield, by polymerisation of an olefin using a catalyst comprising a solid catalyst component and an organometallic compound, the solid catalyst component being prepared by reacting (1) a reaction product obtained by reacting the reaction product of Si oxide and/or A1 oxide and a Ti compound, or a Ti compound and a V compound (Ti.V component) with an organoaluminium compound, with (2) a reaction product obtained by the reaction of a Mg halide, with a compound of the general formula Me(OR) n X z-n where Me represents an element of Groups I to IV, z represents the valence of the element Me, n is O a R² b R³ c Si(OR⁴) d X 4-(a+b+c+d) wherein R¹, R², R³ and R⁴ may be the same or different and are each a hydrocarbon radical having 1 to 20 carbon atoms, X is a halogen atom and a,b,c and d are O ≦ a