As a model for the splitting of pyrimidine base pair defects by DNA photolyase, the uracil dimer and its anion radical have been studied by ab initio HF and MP2 methods. A transition state and an intermediate of the uracil dimer anion radical cleavage reaction have been located. The activation barrier for breaking the C5-C5' bond in the dimer is found to be 6.2 kcal/mol at the UHF level. However, an MP2 single-point energy evaluation shows a vanishing barrier. The overall reaction energy for the dimer splitting is not significantly affected by electron attachment to the neutral dimer. In the dimer anion radical, the transition state, and the intermediate, the two pyrimidine rings differ substantially in their electronic and geometric structure. A one-step mechanism is proposed for the dimer splitting in the DNA photorepair process.