Interfacial shear stress transfer of a monolayer graphene on top of a polymer substrate subjected to uniaxial tension was investigated by a cohesive zone model integrated with a shear-lag model. Strain distribution in the graphene flake was found to behave in three stages in general, bonded, damaged, and debonded, as a result of the interfacial stress transfer. By fitting the cohesive-shear-lag model to our experimental results, the interface properties were identified including interface stiffness (74 Tpa/m), shear strength (0.50 Mpa), and mode II fracture toughness (0.08 N/m). Parametric studies showed that larger interface stiffness and/or shear strength can lead to better stress transfer efficiency, and high fracture toughness can delay debonding from occurring. 3D finite element simulations were performed to capture the interfacial stress transfer in graphene flakes with realistic geometries. The present study can provide valuable insight and design guidelines for enhancing interfacial shear stress transfer in nanocomposites, stretchable electronics and other applications based on graphene and other 2D nanomaterials.