We are reporting an unusual closed-loop phase behavior of poly(ethylene glycol)-beta-poly(ethyl-2-cyanoacrylate) (PEG-PEC) aqueous solutions. As the temperature increased from 0 to 60 degrees C, the aqueous polymer solution (12 wt %) underwent sol-to-gel and gel-to-syneresis transitions. However, the polymer aqueous solution persisted as a sol phase below 4.0 wt % as well as above 16 wt % in the same temperature range, thus forming a closed-loop gel domain in the phase diagram. The closed-loop gel domain is suggested to be a result of the balance between the aggregation and the stabilization of micelles in specific temperature and concentration ranges.