A polyhydroxyalkanoate (PHA) was enzymatically synthesized in vitro, and the end structure of PHA associated with a chain transfer (CT) reaction was investigated. In the CT reaction, PHA chain transfers from PHA synthase (PhaC) to a CT agent, resulting in covalent bonding of CT agent to the PHA chain at its carboxyl end. In vitro CT reaction has never been demonstrated because of relatively low yields of in vitro synthesized poly[(R)-3-hydroxybutyrate)] (P(3HB)), which makes it difficult to characterize the end structures of the polymers by nuclear magnetic resonance (NMR). To overcome these difficulties, a novel in vitro synthesis method that produced relatively larger amounts of P(3HB) was developed by employing PhaC(Da) from Delftia acidovorans and two enantioselective enoyl-coenzyme A (CoA) hydratases which were R-hydratase (PhaJ(Ac)) from Aeromonas caviae and S-hydratase (FadB1x) from Pseudomonas putida KT2440 with beta-butyrolactone and CoA as starting materials. Using this method, P(3HB) synthesis was performed with tetraethylene glycols (TEGs) as a discriminable CT agent, and the resultant P(3HB) was characterized by H-1-NMR. NMR analysis revealed that the carboxylic end of P(3HB) was covalently linked to TEGs, providing the first direct evidence of in vitro CT reaction.