The invention relates to a process for grafting hydrolysable silane groups to a polyolefin, comprising reacting the polyolefin with an unsaturated silane, containing an olefinic —C═C— bond or acetylenic —C≡C— bond and having at least one hydrolysable group bonded to Si, or a hydrolysate thereof, in the presence of means capable of generating free radical sites in the polymer. The silane contains an aromatic ring or a further olefinic double bond or acetylenic unsaturation, the aromatic ring or the further olefinic double bond or acetylenic unsaturation being conjugated with the olefinic —C═C— or acetylenic —C≡C— unsaturation of the silane. The unsaturated silane may also contains electron-withdrawing moiety with respect to the olefinic —C═C— or acetylenic —C≡C— bond. The invention permits to provide a silane-modified polyolefin having a high grafting efficiency while limiting/preventing polymer degradation by chain scission. The silane-modified polyolefin can be further reacted with a polar surface, a filler or a polar polymer or reacted on itself to crosslink the polyolefin and obtain enhanced physical properties of the composites made thereof.