A modified polyolefin prepared from an aqueous suspension containing (a) a crystalline polyolefin, (b) 1 to 500 parts by weight of a vinyl monomer per 100 parts by weight of the crystalline polyolefin, and (c) 0.01 to 10 parts by weight of a radical polymerization initiator per 100 parts by weight of the vinyl monomer by impregnating the crystalline polyolefin (a) with the vinyl monomer (b) and heating the aqueous suspension to polymerize the vinyl monomer (b) at a temperature equal to or higher than a temperature at which the crystal region of the polyolefin (a) begins to substantially melt, which has an excellent effect of improving processability of other polyolefins and provides a polyolefin composition having excellent properties such as processability, impact resistance, rigidity and surface properties when the modified polyolefin is incorporated into polyolefins optionally with a core-shell graft copolymer and/or an inorganic filler.