Polyolefins, such as polyethylene and polypropylene, are low-surface-energy materials that are poorly adhesive owing to weak van der Waals (VDW) forces. While physical bonding offers an attractive alternative to conventional VDW-based adhesion, the development of adhesion methods based on this mechanism requires an understanding of the fracture mechanism of adhesive interfaces. Herein, we reveal the fracture mechanism associated with “nailing adhesion,” in which two polymers are tightly bound physically by needle-like polyolefin crystals at a polyurethane/polyolefin adhesive interface. The mechanism was directly observed by transmission electron microscopy using the modified copper-grid technique. The peel strength and fracture mechanism depended on the polyolefin type (polyethylene or polypropylene). During crack growth at the adhesive interface, needle-like polyethylene crystals deform and finally break, whereas needle-like polypropylene crystals snap with crazing. The deformation of needle-like polyethylene crystals enhances peel strength more than the crazing and snapping of needle-like polypropylene crystals.