Geometrical optimization of nanowire arrays (NWAs) has been regarded as a straightforward and important route to improve the performance for photoelectrocatalytic systems but has not been realized with copper oxides yet. In this work we successfully performed the control to CuO NWAs via electrochemically prepared Cu(OH) 2 intermediate structures. The arrays consist of uniform nanowires with tunable length from 2 to 10 μm and aspect ratios over 100. Results suggest that the optimization can significantly improve the photocurrents several times, which is mostly due to a quantum efficiency over 57% (455 nm) in the presence of electron scavengers. Notably diverged charge transport and transfer between NWAs were revealed by photoelectrochemical impedance and Mott−Schottky measurements, which suggest the necessity of suitable structure for the better charge separation and collection. Moreover, our protocol can be adapted to prepare Cu 2 O NWAs, which can provide great potential for developing highperformance photocathodes in the future.