Abstract: DNA is damaged by various oxidative stresses. Oxidized DNA can increase mutations and the risk of cancer. Guanine is highly sensitive to several oxidative stresses due to its low oxidation potential. It is known that 2,2,4-triamino-5(2H)-oxazolone (Oz), iminoallantoin (Ia) and spiroiminodihydantoin (Sp) are oxidized guanine lesions. These oxidized bases can be paired with guanine and cause G:C-C:G transversions. Although our previous results showed more effective incorporation of guanine opposite Oz compared to that opposite Ia or Sp (K. Kino et al., ChemBioChem, 2009), Oz:G base pair was less stable than Ia:G or Sp:G base pair by ab initio calculation (M. Suzuki et al., Molecules, 2012). Oz:G forms two hydrogen bonds and is planar, while Ia:G and Sp:G have three hydrogen bonds and are nonplanar. This difference can be due to the fact that Ia:G and Sp:G contain a sp³ carbon but not Oz:G. Since stacking planar base pairs in DNA significantly enhances the stability of DNA duplex, we investigate whether the stability of DNA duplex containing the oxidized lesion can be accounted for by the stacking effect in this study. The structure of DNA duplex containing Oz:G was the most stable. This our data agreed with the previous results that translesion synthesis across Oz was more effective than Ia or Sp.