MgO nanowires with uniform diameter distribution of-about 20 nm have been synthesized by boron oxide-assisted catalytic growth method. X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy were used to characterize the structure and defects. The results indicate that MgO nanowires are single-crystal face-centered cubic structures with the growth axis of [100] for straight nanowires. The bending of MgO nanowires could also be observed and explained based on the switching of growth direction from [106] to [201] and to [301]. The contamination from the reactant boron oxide results in the rectangular domain defects. The growth mechanism was also proposed for the synthesis of MgO nanowires.