Hydration of uranyl ion (UO22+) in aqueous system was investigated using hybrid density functional theory B3LYP method. Gibbs free energies and solvation energies for different UO2(H2O)(n)(2+) (n=4, 5, and 6) (including the first and the second solvation shell) clusters were calculated. Polarized continuum model (PCM) was used to calculate the solvation energy. The results show that the hydration number of UO22+ in liquid phase is 5. The hydrolysis reactions of UO2(H2O)(5)(2+) were investigated. The temperature and pressure effects on the reaction energies were studied. It was found that temperature and pressure significantly changed the equilibrium constants of hydrolysis reactions.