Evidences of domain formation in amorphous aromatic polyester (U-polymer) during annealing at temperature below its Tg (about 190℃) for 8 to 120 hours were given by TEM，SAXS and DSC studies in this work．The USe of golddecoration technique made the morphology of samples more distinct．Kratky small angle X-ray collimation system was adopted to remove most of the parasitic scattering due to slit edges，thus im-proving the resolution．In order to confirm that it was the fusion of ordered areas in annealed samples that caused the abnormal endothermic peaks on DSC curves．different heating rates were used for DSC measurements．The excess enthalpy (△H)o was ob-rained by extrapolating the heating rate to zero．From the resuits obtained by TEM，SAXS and DSC．it was shown that domains with long spacing of 40-一60 nm were for reed．when the polymer was annealed for a long Period of time at 10一15℃ below its Tg.