Poly(caprolactone-b-ethylene glycol-b-caprolactone) (PCL-PEG-PCL) triblock copolymer aqueous solution (> 15 wt %) undergoes the sol-gel-sol transition as the temperature increases from 10 to 60 degrees C. The mechanism and structure-property relationship of the sol-gel transition were investigated. In particular, compared with the PEG-PCL-PEG triblock copolymers recently reported by our group, PCL-PEG-PCL has (1) a synthetic advantage without a hexamethylene diisocyanate coupling step, (2) a wider gel window of over 15-32 wt %, and (3) a larger gel modulus. Both PEG-PCL-PEG and PCL-PEG-PCL polymers are an important progress in the biodegradable thermogelling system in that they can be lyophilized in a powder form, are easy to handle, are easy to redissolve to a clear solution, and show little syneresis through the gel phase.