Abstract <p>As a sustainable and environmentally friendly reinforcing material, nanocellulose (NC) is characterized by a high aspect ratio, a large specific surface area, low density, and high mechanical strength. These attributes make NC particularly effective at enhancing the mechanical performance of polyolefins (PO). However, the thermodynamic incompatibility between hydrophilic NC and hydrophobic PO, along with the process-induced morphology evolution, critically influences these properties. This review goes beyond a descriptive compilation to provide a critical synthesis of the physicochemical mechanisms governing these composites. We begin by contrasting the classification and preparation methods of NC, with particular emphasis on distinguishing microfibrillated and nanofibrillated cellulose, as well as the inclusion of bacterial nanocellulose. Recent progress in surface modification is analyzed through thermodynamic frameworks to elucidate dispersion stability and the prevention of hornification. Crucially, to address the limitation of stress transfer in these hydrophilic-hydrophobic systems, we provide an in-depth analysis of interfacial micromechanics. Following this, the advantages and disadvantages of various NC/PO composite preparation methods are critically compared in the context of micromechanical insights, spanning laboratory-scale solvent casting to industrially viable melt compounding techniques. Special attention is given to the Kyoto process and the mechanistic role of solvent exchange in mitigating capillary-induced aggregation. Additionally, existing challenges in PO/NC composite research are discussed, and future research outlooks regarding scalability and recyclability are proposed. The significance of this review lies in providing a comprehensive understanding of NC/PO composite advancements, thereby guiding future studies and applications in this field.</p> Graphical Abstract <p></p>