In the current study, we have focused on isolation and detection of major radical oxidation products from theaflavin in order to better understand antioxidation mechanisms of this compound. Theanaphthoquinone was identified as a major oxidation product of theaflavin from two different oxidant model systems: DPPH and peroxidase/hydrogen peroxide. This result indicated that the benzotropolone moiety in theaflavin may play an important role in its antioxidant properties. The stability of theaflavin was studied in varying pH solutions: simulated gastric juice and buffer solutions of pH 5.5, pH 7.4, and pH 8.5. The results indicated that theaflavin is unstable in alkaline conditions, while it was stable in acidic conditions. Theanaphthoquinone was identified as an autoxidation product of theaflavin during its stability study in alkaline conditions.