The invention provides crystals of P<；1>；,P<；4>；-di(uridine 5'-)tetraphosphate or a salt thereof； a process for producing the crystals； and a process for producing P<；1>；,P<；4>；-di(uridine 5'-)tetraphosphate (U2P4) or a salt thereof from UMP serving as a starting material and by use of DPC and PPi, which process comprises at least one of the following treatment steps: (a) adding UMP diphenylphosphate (UMP-DPP) in divided portions during a step of reaction of UMP-DPP with a PPi-organic alkali salt； (b) carrying out reaction of UMP-DPP with a PPi-organic alkali salt in the presence of a base； and (c) further treating the synthesized U2P4 with an alkali. The crystals of U2P4 or a salt thereof obtained through the process according to the invention have high purity and stability and a less hygroscopicity as compared with a lyophilized product, to thereby serve as a useful raw material for preparing a pharmaceutical. The process for producing U2P4 or a salt thereof according to the invention realizes high yield and enables large-scale synthesis.
