Nitrogen-doped p-type zinc oxide (p-ZnO:N) thin films were fabricated on a-/c-plane sapphire (a-/c-AlO) by plasma-assisted molecular beam epitaxy. Hall-effect measurements show that the p-type ZnO:N on c-AlO degenerated into n-type after a preservation time; however, the one grown on a-AlO showed good stability. The conversion of conductivity in the one grown on c-AlO ascribed to the faster disappearance of N and the growing N , which is demonstrated by x-ray photoelectron spectroscopy (XPS). Compressive stress, caused by lattice misfit, was revealed by Raman spectra and optical absorption spectra, and it was regarded as the root of the instability in ZnO:N.