ZnO nanowires have been self-assembled on amorphous carbon using the thermal chemical vapor transport and condensation without any metal catalysts. The temperature dependence of the growth rate of ZnO nanowires is characterized, and the growth activation energy is estimated to be 234 kJ/mol based on the experimental data. It is found that the growth rate of the thin ZnO nanowire is smaller than that of the thick one. A size-dependent kinetic model is proposed to address the unusual growth behaviors of ZnO nanowires. The theoretical predictions are consistent with experiments. (C) 2010 Elsevier B. V. All rights reserved.