We have grown ZnO films on Zn-polar and O-polar ZnO substrates pretreated at different temperature by metal-organic chemical vapor deposition. Generally, the Zn-polar samples have better structural and optical properties than the O-polar samples. We also found that the Zn-polar samples have a great performance on the suppression of the unintentionally doped carbon impurities. The sample grown on the 1000 degrees C-pretreated ZnO substrate has clearly showed a step-flow growth mode with the surface roughness as low as 2.654 nm, which could be regarded as the best growth condition. (C) 2015 Elsevier B.V. All rights reserved.