MgO nanostructures with different morphologies were fabricated by regulating the composition of raw materials, using the chemical vapor deposition method with gold as a catalyst. Polycrystalline MgO nanopillars were prepared using only Mg3N2 as the raw material. Single crystalline nanowire arrays can be obtained by adding carbon powder into the raw material. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy were used for the detailed characterization of (100) nanowire arrays. Besides the vertical growth direction [100], the nanowires change into two kinds of orientations after growing to a certain length: 〈111〉 and 〈001〉. The epitaxial growth was attributed to the lattice match and regulation of the concentration of the Mg source. The variation of growth orientations was probably caused by the difference in surface energy and reaction temperature.