Radial heterojunction nanowires (NWs) using a ZnO(shell)/Si(core) coaxial structure are for the first time reported for a novel photodiode application. Strong antireflective characteristics are shown by employing vertically aligned Si NW arrays. A thin ZnO shell deposited onto a Si NW formed a radial junction which enabled effective separation of charge carriers. Furthermore, a ZnO nanostructure demonstrates a very high internal gain in photoconductivity due to the surface-enhanced electron-hole separation. The photodetection range, either ultraviolet (UV) or visible, can be determined by applying forward or reverse bias, respectively. Compared to a planar heterojunction photodiode, a photoresponsivity of the radial heterojunction structure shows similar values despite only ∼20% consumption of a ZnO thickness required for a planar junction. In addition, ∼2.5 times increase in UV responsivity is also presented using the radial heterojunction structure under the ZnO thickness same as a planar counterpart. Our coaxial ZnO/Si NW photodetectors suggest bright prospect for enhancing a photoresponsivity while less consuming ZnO via controlling the wired nanostructure.