Abstract The ability to tailor interfacial shear strength for a particular fiber and resin system is critical to the development of composite materials that perform optimally in specific applications. One approach to tailor the interface is to introduce a secondary phase between the fiber and matrix, which can act to functionally grade the material properties and enhance load transfer across the interface. This approach has been applied in the past using nanowires, nanotubes, and whiskers and was demonstrated to significantly enhance interface performance. Unfortunately, these processes lack control over the interphase morphology to allow design of the interface for optimal properties. Recently, ZnO nanowires grown on the surface of carbon fibers have demonstrated more than a 110% increase in interfacial strength . Unlike other treatments, this interfacial reinforcement allows precise morphology control. Here, we develop the parameters for the growth of nanowires with varying lengths and diameters and study the influence of the nanowire’s morphology on the interfacial shear strength. ZnO nanowire arrays are grown on carbon fibers, with nanowire diameters ranging from 50 to 200 nm and lengths up to 4 μm. The interfacial shear strength with varying nanowire dimensions is shown to increase by up to 228%, ranging from 45.72 to 154.64 MPa. Unlike existing whiskerization approaches, it is shown that the tensile strength of the ZnO nanowire coated fibers remains constant throughout all growth procedures. The development of an interphase offering control over the interface strength and toughness will provide a means to produce multifunctional composites with optimized performance for multiple applications.