Poly(3-hydroxybytyrate-co-3-hydroxypropionate), poly(3HB-co-3HP), is a possible alternative to synthetic polymers such as polypropylene, polystyrene and polyethylene due to its low crystallinity and fragility. We already reported that recombinant strains of Shimwellia blattae expressing 1,3-propanediol dehydrogenase DhaT as well as aldehyde dehydrogenase AldD of Pseudomonas putida KT2442, propionate-CoA transferase Pct of Clostridium propionicum X2 and PHA synthase PhaC1 of Ralstonia eutropha H16 are able to accumulate up to 14.5% (wtPHA/wtCDW) of poly(3-hydroxypropionate), poly(3HP), homopolymer from glycerol as a sole carbon source (Appl Microbiol Biotechnol 98:7409-7422, 2014a). However, the cell density was rather low. In this study, we optimized the medium aiming at a more efficient PHA synthesis, and we engineered a S. blattae strain accumulating poly(3HB-co-3HP) with varying contents of the constituent 3-hydroxypropionate (3HP) depending on the cultivation conditions. Consequently, 7.12, 0.77 and 0.32 gPHA/L of poly(3HB-co-3HP) containing 2.1, 8.3 and 18.1 mol% 3HP under anaerobic/aerobic (the first 24 hours under anaerobic condition, thereafter, aerobic condition), low aeration/agitation (the minimum stirring rate required in medium mixing and small amount of aeration) and anaerobic conditions (the minimum stirring rate required in medium mixing without aeration), respectively, were synthesized from glycerol by the genetically modified S. blattae ATCC33430 strains in optimized culture medium.