In the search for a new thermogelling biomaterial, we are reporting polyethylene glycol)-poly(trimethylene carbonate) (PEG-PTMC) diblock copolymers. The PEG-PTMC (550-2750) diblock copolymer aqueous solutions (>25 wt %) underwent sol-to-gel-to-syneresis (macroscopic phase separation between the polymer and water) transition as the temperature increased. The sol-to-gel transition temperature could be controlled in a range of 20-75°C by varying polymer concentration in water, molecular weight, and composition of the polymer. 13C NMR spectra, a transmission electron microscopic image, and dynamic light scattering suggest that the micellar aggregation through the dehydration of PEG is involved in the sol-to-gel transition. The in-situ gel formation was confirmed by subcutaneous injection of the polymer aqueous solution (30 wt %, 0.5 mL) into rats. Because of the diffusion of the polymer with a low molecular weight out of the gel, mass loss of 15 wt % was observed over 20 days in in-vivo study. On the other hand, the polymer is so stable in phosphate buffer saline that any changes in molecular weight, pH, and gel mass were not observed over the same period of time in in-vitro study.