The aqueous solutions of poly(ethylene glycol) grafted with poly(lactic acid-co-glycolic acid) flow freely at room temperature but form gels at higher temperature. The existence of micelles in water at low polymer concentration was confirmed by cryo-transmission electron microscopy and dye solubilization studies. The micellar diameter is about 9 nm, and the critical micelle concentration is in a range of 0.01-0.05 wt %. The critical gel concentration, above which a gel phase appears, was 16 wt %, and the sol-to-gel transition temperature was slightly affected by the concentration between 16 and 25 wt %. At sol-to-gel transition, viscosity and modulus increased abruptly, and C-13 NMR showed molecular motion of hydrophilic poly(ethylene glycol) backbones decreased while that of hydrophobic poly(lactic acid-co-glycolic acid) side chains increased. The hydrogel of PEG-g-PLGA with hydrophilic backbones was transparent during degradation and remained a gel for 1 week,suggesting a promising material for shortterm drug delivery.