Carbon impurities are usually unintentionally doped in N-doped ZnO when attempting to realize p-type conductivity by metal-organic chemical vapor deposition. Mn-N co-doping technique, which is developed to realize hole-mediated room temperature ferromagnetism in ZnO, may further enhance the carbon incorporation. In this work, two kinds of Mn-N co-doped samples, grown at low temperature (400 degrees C) and high temperature (600 degrees C), respectively, have been compared to study the influences of carbon impurities. In contrast to that found on N mono-doped ZnO, an enhanced incorporation of carbon impurities is observed in the high-temperature-grown Mn-N co-doped sample with the conductivity changed from p to n type. According to X-ray photoelectron spectroscopy measurement, the compensation effect from carbon impurities is applied to elucidate the origin of the conductivity transition. Correspondingly, superconducting quantum interference device measurement certainly shows a much smaller value of the saturation magnetization for the high-temperature-grown sample. A possible effect from carbon related complexes on magnetization is also proposed to explain the decrease of the magnetic moment from the view of weakened spin polarization induced by unintentionally incorporated carbon. This assumption is further supported by the first-principle calculation on the Mn-N co-doped ZnO system with carbon incorporation. (C) 2010 Elsevier B.V. All rights reserved.