Three-dimensional ZnO hierarchical structures with high densities of secondary ZnO nanowires grown on primary ZnO fibers have been fabricated employing a co-precipitation reaction growth on the electrospinning PVA/zinc acetate nanofibers template followed by an annealing process in air. The structural analyses showed that as-prepared amorphous ZnC2O4 nanowires converted into wurtzite ZnO after annealing. The morphology of the ZnO hierarchical structures could be further controlled by adjusting the mixing ratio of H2C2O4 to CH3CH2OH. The hierarchical structured ZnO sample shows an enhanced photocatalytic performance compared with the other nanostructured ZnO powders of nanoparticles, and nanofibers, which can be attributed to the special structural feature with an open and secondary ZnO nanowire that significantly facilitates the diffusion and mass transportation of Methylene blue (MB) molecules and oxygen species in photochemical reaction of MB degradation.