Abstract: A given amount of crystalline polypropylene resin is blended with an olefin copolymer rubber to prepare a preblend comprising an olefin copolymer rubber/crystalline polypropylene and then the preblend is melt-blended with a crystalline polypropylene resin in the presence of peroxide. The present invention is based upon the fracture mechanism occurring between the crystalline polypropylene and rubber and is accomplished by the reciprocal combination of the reactions such as cross-linking reaction, random scission reaction of the polymer main chain, graft reaction and blocking reaction to form a cell structure. An object of the present invention is to provide a method which results in a product having better homogeneity, better heat-resistance, higher impact-resistance and greater flowabilty than heretofore known methods of preparation.