To elucidate the mechanism of DNA strand breaks by low-energy electrons (LEE), theoretical investigations of the LEE attachment-induced C(5')-O(5') sigma bond breaking of pyrimidine nucleotides (5'-dCMPH and 5'-dTMPH) were performed by using the B3LYP/DZP++ approach. The results indicate that the pyrimidine nucleotides are able to capture electrons characterized by near-0-eV energy to form electronically stable radical anions in both the gas phase and aqueous solution. The mechanism of the LEE-induced single-strand bond breaking in DNA might involve the attachment of an electron to the bases of DNA and the formation of base-centered radical anions in the first step. Subsequently, these radical anions undergo either C-O or glycosidic bond breaking, yielding neutral ribose radical fragments and the corresponding phosphoric anions or base anions. The C-O bond cleavage is expected to dominate because of its low activation energy. In aqueous solutions, the significant increases in the electron affinities of pyrimidine nucleotides ensure the formation of electronically more stable radical anions of the nucleotides. The low activation energy barriers for the C(5')-O(5') bond breaking predicted in this work are relevant when the counterions are close enough to the phosphate moiety of DNA.