The molar fraction of 3-hydroxyvalerate (3HV) in poly (3-hydroxybutyrate-3-hydroxyvalerate) [P(3HB-3HV)] was modulated through the co-amplification of the phbC gene encoding PHB synthase and the zwf gene encoding glucose-6-phosphate dehydrogenase generating NADPH from the metabolism of fructose as a cofactor in Ralstonia eutropha. The biosynthesis of poly-3-hydroxybutyrate (PHB) and P(3HB-3HV) increased appreciably after the co-amplification of the phbC and zwf genes due to the accelerated polymerization reaction and enhanced NADPH supplementation. In particular, the 3HV molar fraction in P(3HB-3HV) was effectively modulated. pH-stat fed-batch cultivation of the R. eutropha transformant co-amplifying the phbC and zwf genes was also carried out to increase the biosynthesis of P(3HB-3HV) and modulate the 3HV molar fraction. The content and molar fraction of P(3HB-3HV) achieved up to 60.1% and 71.9 mol%, respectively, through the concurrent feeding of valerate and fructose.