The effects of propionic acid feeding strategy on production of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [P(3HB-co-3HV)] were studied with Ralstonia eutropha. Flask culture revealed that the time and concentration of propionic acid addition had significant effects on cell growth, P(3HB-co-3HV) synthesis, and HV fraction in the copolymer. In fed-batch culture, a low ratio of propionic acid to glucose (P/G) led to high dry cell weight (DCW), P(3HB-co-3HV) content and productivity, but low HV unit fraction. A high P/G ratio led to, on the other hand, high HV unit fraction but the low P(3HB-co-3HV) content and productivity. The specific P(3HB-co-3HV) synthetic rate and the specific HV synthetic rate declined in fed-batch cultures, which deteriorated with high P/G feeding due to the inhibitory effect of propionic acid accumulated in the culture broth. According to the non-steady state synthesis of P(3HB-co-3HV) by R. eutropha, an optimal feeding strategy to control the propionic acid accumulation was developed and demonstrated. The propionic acid feeding rate was reduced with time, and DCW, P(3HB-co-3HV) concentration, P(3HB-co-3HV) content and HV fraction in P(3HB-co-3HV) reached 52.1 g l -1 , 40.8 g l -1 , 78.3% and 16.2 mol%, respectively. P(3HB-co-3HV) productivity was 0.74 g l -1 h -1 .