Nanocrystalline ZnO powders, whose crystalline sizes were 14.3, 21.2, 24.1, and 35.3 nm, respectively, were prepared by homogeneous precipitation process. Nanocrystalline ZnO powders were characterized with powder X-ray diffractometer (XRD), transmission electron microscopy (TEM) and X ray photoelectron spectroscopy (XPS). The performance of desulfurization of H2S at room temperature was investigated. Results demonstrated that the smaller the size of nanocrystalline ZnO, the darker the color of desulfurization products would be, which was more similar to the characteristics of multi-sulfide results, indicating that the mechanism of desulfurization of nanocrystalline ZnO was not the same as that of the analytical pure ZnO. Nanocrystalline ZnO improved the performance of desulfurization of H2S at room temperature, and the activity time of nanocrystalline ZnO (14.3 nm) for the desulfurization was 34 times that of analytical pure ZnO at absence of oxygen. When the oxygen was present in reaction, the accuracy of desulfurization was improved, in which H2S was prone to be oxidized into element S, and SO2 was not detected at outlet of the reactor.