Ralstonia eutropha was cultivated under oxidative conditions in the presence of hydrogen peroxide and methyl viologen to stimulate the flux of NADPH to the PHA biosynthesis pathway. The effects of oxidants on the biosynthesis of poly-beta -hydroxybutyrate(PHB) and poly(3-hydroxybutyrate-3-hydroxyvalerate 3HV)] were investigated. The biosynthesis rate and concentrations of PHB and its copolymer P(3HB-3HV) increased significantly under the oxidative pressure of methyl viologen, meanwhile, the molar fraction of 3HV in P(3HB-3HV) remained constant. The effect of methyl viologen on the Bur of the intermediate compounds was investigated by measuring the activity of enzymes related to PHB biosynthesis, glucose 6-phosphate dehydrogenase (G6PD), and NADPH level. The oxidant significantly activated the G6PD of R. eutropha, increasing the NADPH level used for the reduction reaction of acetoacetyl-CoA to beta -hydroxybutyryl-CoA, thereby enhancing the biosynthesis of PHB and P(3HB-3HV).