Abstract: The saturation capacity and reaction probability of O₂ adsorption on clean uranium surface have been quantitatively measured by low pressure PVT method, and their reactivity and kinetic property were obtained. The experimental results show that O₂ exhibited the reaction characteristics of adsorbed gas amount（coverage） is approaching a maximum value, about 4.3×10¹⁵/cm² and reaction probability is decreasing with a rise in adsorbed gas amount. Oxygen molecule on the clean uranium is dissociated rapidly, reacted to form oxide with exterior uranium atom. Oxygen particles diffuse to pass through oxide layer and react with uranium atom at interface. The initial reaction kinetics relies on the impacts of both product layer composition and thickness on the molecular dissociation and dissociative particles diffusion process.