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 °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.