A comparative study of the second harmonic generation in ZnO micro particles and nanorods has been experimentally studied by Kurtz technique using nanosecond pulsed Nd:YAG laser. The results have been theoretically explained by taking into account the quadrupole moment as well as surface nonlinearity. It was observed that the nanorods yield polarized second harmonic signal while the second harmonic signal obtained from the micro particles was unpolarized.