Construction of junctions between semiconductors is an effective way to promote charge separation and thus improve the photoelectrochemical and photocatalytic performance. Specifically, p-n homojunctions are more efficient due to the larger driving force and lower transfer barrier over the interface. Herein, we fabricated ZnO p-n homojunctions by depositing n-type ZnO nanoparticles (with oxygen vacancy) on the surface of p-type ZnO (with metal vacancy). The structures of the composite were well characterized by TEM, XRD, TG, XPS and EPR analyses. Meanwhile, both the "V-shaped" Mott-Schottky plots and anodic shift of onset potentials confirmed the existence of p-n homojunction, which exhibits much more efficient charge transfer and separation than pure type ZnO as indicated by PL and EIS measurements. As a result, p-n homojunction exhibited activity of 3.2-fold and 3.3-fold higher than pure p-ZnO in photodegradation of phenol and as photocathode in photoelectrochemical water splitting, respectively. This work provides a new strategy for the design and fabrication of highly efficient photocatalysts with promoted charge separation.