Cellulose nanocrystals (CNC) are renewable, sustainable, and non-toxic nanomaterials with high surface area, low density, and strength properties several times higher than many metals and polymers. Cellulose nanocrystals, however, are polar and hard to disperse in nonpolar polymers such as polyethylene. Considering that polyethylene is one of the most important commodity polymers today, a significant sector of the polymer market has not benefited yet from the remarkable properties of CNCs. In this article we present a novel method to overcome this difficulty. Our approach was to modify the surface of CNCs with bifunctional organic molecules having terminal vinyl groups that copolymerize, in-situ, with ethylene using a constrained geometry catalyst. These hybrid materials compatibilize the hydrophilic CNC and the hydrophobic polyolefin phases, leading to good dispersion of the CNC phase in the polyethylene matrix.