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Although efforts have been undertaken to produce polyhydroxyalkanoates (PHA) with various monomers, the low yield of PHAs because of complex metabolic pathways and inhibitory substrates remains a major hurdle in their analyses and applications. Therefore, we investigated the feasibility of mass production of PHAs containing 5-hydroxyvalerate (5HV) using δ-valerolactone (DVL) without any pretreatment along with the addition of plant oil to achieve enough biomass. We identified that PhaC_BP-M-CPF4, a PHA synthase, was capable of incorporating 5HV monomers and that <i>C. necator</i> PHB⁻⁴ harboring <i>phaC</i>_BP-M-CPF4 synthesized poly(3HB-<i>co</i>-3HHx-<i>co</i>-5HV) in the presence of bean oil and DVL. In fed-batch fermentation, the supply of bean oil resulted in the synthesis of 49 g/L of poly(3HB-<i>co</i>-3.7 mol% 3HHx-<i>co</i>-5.3 mol%5HV) from 66 g/L of biomass. Thermophysical studies showed that 3HHx was effective in increasing the elongation, whereas 5HV was effective in decreasing the melting point. The contact angles of poly(3HB-<i>co</i>-3HHx-<i>co</i>-5HV) and poly(3HB-<i>co</i>-3HHx) were 109 and 98°, respectively. In addition, the analysis of microbial degradation confirmed that poly(3HB-<i>co</i>-3HHx-<i>co</i>-5HV) degraded more slowly (82% over 7 days) compared to poly(3HB-<i>co</i>-3HHx) (100% over 5 days). Overall, the oil-based fermentation strategy helped produce more PHA, and the mass production of novel PHAs could provide more opportunities to study polymer properties.