The structures of the Zn-O bonding in ZnO (0002) thin films prepared by metal organic chemical vapor deposition have been studied by reflective second harmonic generation (RSHG). The polar Zn-O bond on the top layer is not canceled out and presents 3 mm symmetrical structures on the well-grown ZnO (0002) surface. The average polar strength of the Zn-O bond is correlated with the quality of the ZnO (0002) thin film. The mirror symmetry is caused by the nonvanished polar of twin boundary due to the mismatch between the ZnO film and sapphire substrate and analyzed using s-polarized RSHG with s-polarized fundamental light irradiation. These results demonstrate that the Zn-O heteropolar bonds on the smooth ZnO surface contribute to the SHG intensity.