Biobased poly(gamma-butyrolactone) (P gamma BL) as a fully biodegradable and bioabsorbable biomaterial has shown superior properties compared to those of other aliphatic polyesters. It is of great importance to prepare amphiphilic block copolymer containing P gamma BL block to make ordered nano-objects for biomedical applications such as drug delivery systems. However, such an amphiphilic copolymer containing P gamma BL segment was never successfully prepared mostly due to the synthetic challenges of ring-opening polymerization (ROP) of nonstrained gamma-butyrolactone (gamma BL) monomer. Here, we reported the first preparation of amphiphilic poly(ethylene glycol)-b-poly(gamma-butyrolactone) (PEG-b-P gamma BL) diblock copolymer by using PEG as a macroinitiator. We applied two types of bases to initiate the ROP of gamma BL. An organic cyclic trimeric phosphazene base (CTPB) was first applied to activate the terminal hydroxyl group of PEG as macroinitiator for ROP of gamma BL. On the other hand, sodium hydride was used to activate the hydroxyl group of PEG to form sodium alkoxide as an initiating system for ROP of gamma BL. Both catalytic/initiating system showed moderate control on ROP of gamma BL and successfully produced PEG-b-P gamma BL diblock copolymers with varied molecular weights and relatively narrow molecular weight distributions. The effects of catalytic systems, activation temperatures, and monomer concentrations on gamma BL conversion and molecular weight of PEG-b-P gamma BL were carefully explored. The thermal properties and phase behaviors of obtained PEG-b-P gamma BL were also investigated.