Ab initio CASSCF calculations with the 6-31G basis set are reported for the fragmentation reaction of thymine dimer radical cation. The unpaired electron of thymine dimer radical cation is localized on the lengthened C6-C6' bond, and the fragmentation starts with the cleavage of this bond. The puckering of the cyclobutane ring (C6-C6'-C5'-C5) leads to one of the C6 atoms being pyramidal and the other being planar in the initial thymine dimer radical cation structure. During the C6-C6' bond breaking step, the pyramidal C6 takes on a sp(3) hybridization to accept the localized unpaired electron; the planar C6' takes on a sp(2) hybridization with an empty p orbital. In the next step of the fragmentation, the breaking of the C5-C5' bond, the localized unpaired electron on C6 pairs up with one of the electrons formerly of the C5-C5' bond to form a neutral thymine monomer while the other electron of the C5-C5' bond moves to the other ring to form a thymine radical cation. The reaction proceeds stepwise and exothermally. The puckering of the cyclobutane ring of thymine dimer plays an important role in determining the fragmentation path and the spin distribution at the final stacked pair.