Aqueous solutions of new biodegradable triblock copolymers, poly(ethylene glycol-6-(DLlactic acid-co-glycolic acid)-o-ethylene glycol) (PEG-PLGA-PEG), have shown to have sol-to-gel (lower transition) and gel-to-sol (upper transition) transitions as temperature monotonically increases. The lower transition is important for drug delivery application because the solution flows freely at room temperature and becomes a gel at body temperature. In this paper, the mechanism of gelation was proposed, and the structure-property relationship of the sol-gel transition was investigated. The lower transition may be related to micellar growth and intra- and intermicelle phase mixing and packing, while the upper transition involves breakage of micellar structure. Critical gel concentration and critical gel temperature are controlled by polymer molecular parameters, such as block length and composition of PEG-PLGA-PEG triblock copolymers, and additives, such as salts.