Poly(3-hydroxyalkanoate)s (PHAs) are a class of polymers receiving attention because of their potential as renewable, biodegradable and high-technology properties. Unlike most short chain length (scl) PHAs such as poly(3-hydroxybutyrate) (PHB), medium chain length (mcl) PHAs such as poly(3-hydroxyoctanoate) (PHO) exhibit low crystallinity and are elastomeric in character. PHB-b-PEG-b-PHO block copolymers can combine both properties in block copolymer matrix. In this study, we report the synthesis of the block copolymers combining the PHB and PHO blocks. Transamidation reactions of PHB with polyethylene glycol with primary amine yield equimolar amounts and PHB with amine ends. PHO reacts with the modified PHB containing the amine end to give PHB-b-PEG-b-PHO block copolymers. Structural analysis of the products was performed by using H-1-, C-13, heteronuclear single quantum coherence NMR techniques. Thermal and mechanical properties of the block polymers were also evaluated.