Abstract Hydrothermal method is employed to synthesize nano ZnO particles in solutions containing different surfactant at room temperature and 50 °C under ultrasonic condition. The reactants used are aqueous solutions of zinc chloride and potassium hydroxide, and the surfactants are cetyltrimethylammonium bromide (CTAB) and triethanolamine (TEA). Instruments including synchrotron-based X-ray absorption spectroscope (XAS), Brunauer–Emmett–Teller (BET) surface meter, X-ray diffraction (XRD) spectrometer, and transmission electron microscope (TEM) are used to characterize the properties of the reaction product. All synthesized ZnO samples are in the form of zincite. The particle size of the nano ZnO powders synthesized at room temperature is approximately 46–61 nm (i.e., corresponding to BET surface areas 17.5–23.0 m 2 g −1 ). ZnO particle size can be effectively reduced to approximately 28–39 nm (i.e., corresponding to BET surface areas 27.4–38.9 m 2 g −1 ) if the synthesis processes in the ultrasonic water bath are kept at 50 °C. Moreover, the ZnO powder dispersion is considerably improved when the synthesis is carried out at 50 °C. The intensities of white bands in the X-ray absorption near-edge structure (XANES) spectra from all nano ZnO samples are relatively smaller than that from bulk ZnO. The position (i.e., photon energy (eV)) of the white band peak in the XANES spectrum from the nano ZnO synthesized under CTAB surfactant at 50 °C dramatically shifts to a greater value by approximately +3.6 eV, as compared with those from all other nano ZnO samples, indicating unique electronic state of this sample.