Milled sunflower husk was added to three types of polyolefins (polypropylene, low-density and high-density polyethylene) in a wide composition range. Composite series were prepared with or without maleic anhydride-grafted polyolefin coupling agents to investigate the effect of coupling on the mechanical properties and micromechanical deformation processes. Several independent approaches were followed to estimate interfacial adhesion qualitatively and quantitatively, respectively. The results show that interfacial adhesion is significantly improved in the presence of a coupling agent. As a result, coupling changes the dominant deformation process from matrix/filler debonding to the fracture of sunflower husk particles. Milled sunflower husk can enhance significantly the modulus of polyolefins; however, elongation-at-break values are very small, especially at larger filler content, which represents the largest obstacle of practical application.