In this letter, we report that vertically well-aligned ZnO nanowires were grown on GaN epilayers and c-plane sapphire via a vapor-liquid-solid process by introducing a 3 nm Au thin film as a catalyst. In our experiments, epitaxially grown ZnO nanowires on Au-coated GaN were vertically well-aligned, while nanowires normally tilted from the surface when grown on Au-coated c-$Al_2O_3$ substrates. However, pre-growth annealing of the Au thin layer on c-$Al_2O_3$ resulted in the growth of well-aligned nanowires in a normal surface direction. High-resolution transmission electron microscopy measurements showed that the grown nanowires have a hexagonal c-axis orientation with a single-crystalline structure.