Structures of the two-dimensional atomic nuclei on ZnO (0001)-Zn and (0001)-O polar surfaces were studied by first principles density functional theory. The polarity-dependent nucleation dynamics was investigated by simulating two-dimensional (2D) nuclei consisting of 1-8 ZnO monomers on both polar surfaces. According to total energy calculations, average binding energy per ZnO monomer of the surface nuclei was analyzed to investigate if the nucleation and growth will proceed reasonably in physics. We found nucleation on (0001)-Zn surface was easier than that on (0001)-O surface. By using atomistic thermodynamics analysis, we calculated the Gibbs free energy of formation of these nuclei and made a comparison between the two polar surfaces. On (0001)-Zn surface, the critical Gibbs free energy of formation is much lower than that on (0001)-O surface under the same supersaturation, which leads to a much larger ZnO growth rate and rougher morphology, in accordance with experimental results. In addition, energetic analysis of nucleation at real thermodynamic conditions was achieved by introducing the temperature-and pressure-dependent chemical potentials of ZnO precursors. (C) 2017 Elsevier B.V. All rights reserved.