As the potential applications of nanostructured Si/Cu2O heterojunction in electronic and photoelectronic devices, various methods has been developed to fabricate such heterojunctions. In this study, the Si/Cu2O heterojunction nanowire arrays were fabricated by the in-situ thermal decomposition of Cu(NO3)2 on silicon nanowires. The as-decomposed Cu2O nanoparticles were found to be uniformly distributed on the surfaces of Si nanowires. The microstructure, chemical composition and states of the products were investigated. Utilized as photoanodes for photoelectrochemical water splitting, the Si/Cu2O heterojunction nanowire arrays exhibited a maximum photocurrent density of 4 mA/cm2 at 1.23 V vs. RHE and an onset potential of 0.62 V vs. RHE.