The aqueous solution of poly(ethylene glycol)-poly(caprolactone)-pol),(ethylene glycol) (PEG-PCL-PEG) triblock copolymers (>15. wt. %) undergoing "clear sol - gel - turbid sol" transition as the temperature increases from 20 to 60 °C has been developed. Light scattering and 13C NMR study suggested that the transition mechanisms are the micellar aggregation for the clear sol to gel transition (lower transition), whereas the increase in PCL molecular motion for gel to turbid sol transition (upper transition). In contrast to the previous thermogelling biodegradable polymers with a sticky paste morphology, the powder form of the PEG-PCL-PEG triblock copolymers makes it easy to handle and allows fast dissolution in water. Therefore, the lyophilization into a powder form followed by facile reconstitution was possible. This system is believed to be promising for drug delivery, cell therapy, and tissue engineering.