Tumor hypoxia, either pre-existing or as a result of oxygen bleaching during Photodynamic Therapy (PDT) light irradiation, can significantly reduce the effectiveness of PDT induced cell killing. To overcome the effect of tumor hypoxia and improve tumor cell killing, we propose using supplemental hyperoxygenation during Photofrin PDT. Our previous study has demonstrated that, in an in vivo model, tumor control can be improved by normobaric or hyperbaric 100% oxygen supply. The mechanism for the tumor cure enhancement of the hyperoxygenation-PDT combined therapy is investigated in this study by using an in vivo/in vitro technique. A hypoxic tumor model was established by implanting mammary adenocarcinoma (MCA) in hind legs of C3H mice. Light irradiation (200 J/cm2 at either 75 or 150 mW/cm2), under various oxygen supplemental conditions (room air or carbogen or 100% normobaric or hyperbaric 100% oxygen), was delivered through an optical fiber with a microlens to animals who received 12.5 mg/kg Photofrin 24 hours prior to light irradiation. Tumors treated with PDT were harvested and grown in vitro for colony formation analysis. Treated tumors were also analyzed histologically. The results show that, when combined with hyperoxygenation, the cell killing rate immediately after a PDT treatment is significantly improved over that treated without hyperoxygenation, suggesting an enhanced direct cell killing. This study further confirms our earlier observation that when a PDT treatment is combined with hyperoxygenation, it can be more effective in controlling hypoxic tumors. H&E stain revealed that PDT induced tumor necrosis and hemorrhage. In conclusion, by using an in vivo/in vitro assay, we have shown that PDT combined with hyper-oxygenation can enhance direct cell killing and improve tumor cure.