Abstract: Nanometer sized ZnO with average particle diameters of 15.4 nm, 19.1 nm, 22.9 nm and 33 nm were prepared by calcination at different temperatures. Their desulfurization behavior and selective oxidation of H2S at room temperature were studied, and compared with commercial ZnO. Properties of nanometer ZnO before and after reactions were characterized by XRD and XPS. XRD results show that there is not ZnS peak detected after reaction, and in the presence of oxygen sulfur and one unknown product are formed, but without oxygen two unknown products are found. In the absence of oxygen, the desulfurization abilities of ZnO decrease, and the product color changes from brown, orange to yellow with the increase of ZnO diameter. The brown and orange products are unstable in air. In the presence of oxygen, however, the desulfurization mechanism of nanometer sized ZnO is obviously different. Nanometer sized ZnO can catalyze the oxidation of H2S to sulfur at room temperature. ZnO particle size also gives great influence on this process.