Two shallow donors are observed in sulfur-ion implanted ZnO epitaxial films on sapphire substrates. The resistivity varies from ~10 3 Ωcm for un-implanted samples to 1.7×10 −2 Ωcm for as-implanted ones. This low resistivity is attributed to zinc interstitial (Zn i ), corresponding to the activation energy (26meV) estimated from the temperature dependence of electron concentration. The presence of Zn i is evaluated from ion channeling by Rutherford backscattering spectroscopy. The disordered concentration is ~40% of Zn atoms at a maximum point (300nm) from the surface in as-implanted samples and recovers to ~80% after 1000°C annealing. In 1000°C annealed samples, a shallower donor level of 12meV is observed, suggesting the presence of complex defects consisting of remaining Zn i and sulfur atoms replaced to the oxygen atom.