In situ epitaxial growth of high-oriented ZnO nanowires on annealed zinc film was realized in an environmental scanning electron microscope. Energy dispersive spectra, X-ray diffraction and photoluminescence measurements demonstrated that a thin layer of [001] ZnO was formed on the surface of zinc film after annealing. Formation of a ZnO layer on annealed zinc film caused dense [001] oriented nanowires compared to the sparse random nanowires grown on unannealed zinc film. Possible growth mechanism for the nanowires grown on annealed and unannealed film was discussed. This work offers a novel approach for effective epitaxial growth of high-oriented ZnO nanowires and helps to understand the growth mechanism of ZnO nanowires.