Amphiphilic poly[ (e-caprolactone) -co-glycolide] - block-poly(ethylene glycol) -block-poly[(,-caprolactone)-co-glycolide) [P(CL-GL)-PEG-P(CL-GL)] triblock copolymers with different average lengths of caproyl sequences (Lc,) were synthesized by ring-opening polymerization at different temperatures. A 25% aqueous solution of the copolymer with LCL = 11-0 formed a gel, owing to strong crystallinity-induced hydrophobicity at low temperature, and underwent a gel-sol transition (UCST behavior) when the temperature was increased to 40'C. In contrast, the solution of copolymer with LcL = 6.7 underwent a sol-gel transition (LCST behavior) due to micelle aggregation. However, a clear sol-turbid sol phase transition was observed for the copolymer with more random microstructures (LCL = 5.2).