Process for preparing polyolefins

There is provided a process of preparing a polyolefin by homopolymerizing or copolymerizing an olefin or olefins substantially in a vapor phase condition and in the presence of a catalyst comprising a solid catalyst component and an organometallic compound, the solid catalyst component containing magnesium, one or more elements selected from titanium and vanadium and also containing halogen, in the presence of a polyolefin serving as a medium for dispersing the catalyst and to start homopolymerization or copolymerization of the olefin(s), a solid catalyst component satisfying the above condition and having an average particle diameter substantially larger than that of the solid catalyst component to be used in a principal stage of the polymerization reaction is used at least until an olefin homopolymer or copolymer is formed in an amount equal to the amount of the polyolefin used as the catalyst dispersing medium.