We employed a radical beam assisted deposition technique to prepare single-crystal niobium nitride thin films on MgO(100) substrates. The radical beam containing excited species of nitrogen was produced by an electron cyclotron resonance plasma source and used to irradiate the growing Nb film which was deposited simultaneously by an electron-gun vapour source. The nitride film was found to grow epitaxially on the substrate heated to 600-650 degrees C. The resulting NbN film formed had predominantly B1 structure, resistivity of 44 mu Omega cm at 20 K, and almost equiatomic composition.