MgO nanobelts were obtained by the decomposition of magnesium nitrate in a direct current arc plasma jet chemical vapor deposition process, with a Mo substrate at 950 °C for different durations. The products formed in the first 0.5 min were nanobelts, some of which possessed spherical particles on tips of nanobelts, forming “tadpole-like” nanostructures. When the reaction time was extended to 1 min, nanobelts become longer although spherical particles were still observed. When the reaction time was 3 min, nanobelts became even longer, and no spherical particles were observed. Extending the reaction time to 5 min significantly widened the nanobelts, with widths ranging from 10 to 100 nm. The MgO nanobelt growth occurred along the 〈100〉 and the 〈111〉 axes. The growth mechanism of the MgO nanobelts was a combination of axial VLS and side VS growth. Nanobelts exhibited strong ultraviolet–blue photoluminescence emission under excitation at 350 nm.