The G-selectivity for Cu(l)-mediated one-electron oxidation of 5'-TG(1)G(2)G(3)-3' and 5'-CG(1)G(2)G(3)-3' has been examined by ab initio molecular orbital calculations. It was confirmed that G(1) is selectively damaged by Cu(l) ion for both 5'-TG(1)G(2)G(3)-3'and 5'-CG(1)G(2)G(3)-3', being good agreement with experimental results. The Cu(l)-mediated G(1)-selectivity is primarily due to the stability of the Cu(l)-coordinated complex, [-XG(1)G(2)G(3)-,-Cu(l)(H2O)(3)](+). The Cu(l) ion coordinates selectively to N7 of G(2) of 5'-G(1)G(2)G(3)-3' rather than N7 of G(1). The G(2)-selective coordination induces the G(1)-selective trap of a hole that is created by one-electron oxidation and migrates to GGG triplet. Therefore, the radical cation of G(1) is selectively created in both 5'-TG(1)G(2)G(3)-3' and 5'-CG(1)G(2)G(3)-3', giving the G(1)-selective damage of 5'-G(1)G(2)G(3)-3'.