We present an empirical model based on the solubility of zinc and temperature to explain the observed variations in density and length of hydrothermally grown zinc oxide (ZnO) nanorods on GaN substrate as reported by Le et al. [J. Cryst. Growth, 293, 36 (2006)]. The ionic equilibrium of the growth solution is solved numerically. By using the experimental data and calculated equilibrium concentrations of species in the solution, expressions were formulated to represent the dependence of the ZnO nanorod density and length on the solubility of zinc and temperature. Activation energies of 0.77 and -2.11 eV were found for the length and density, respectively. The proposed model can be used to predict the ZnO nanorod density and length on GaN substrate within the growth temperature range of 333 to 423 K, precursor solution pH between 9.7 and 10.6, and a degree of saturation 20 < S < 60. (c) 2007 The Electrochemical Society.