Abstract: Composites from a semi-crystalline polyolefin and nanometer sized fillers with improved modulus, tensile strength and crack growth resistance are disclosed. The nanometer sized fillers are first functionalized with an aminosilane. Thereafter, a carboxylated or maleated polyolefin is grafted to the filler through an amine-carboxyl reaction. The resulting modified filler is dispersed in a semi-crystalline polyolefin (eg. polyethylene or polypropylene). Co-crystallization between the carboxylated or maleated polyolefin and the semi-crystalline polyolefin can improve interaction between the filler and semi-crystalline polyolefin.