Abstract: Microdomain morphology in styrene-butadiene-styrene triblock copolymer (SBS) and polystyrene (PS) blends was investigated by small angle X-ray scattering. The homopolystyrenes having molecular weight of 500 and 12000,respectively, were blended with SBS up to a concentration of 25%.As the homopolymer concentration increased,the size of the PS microdomains increased.The size of polybutadiene (PB)microdomains,on the other hand,decreased if the added polystyrene was with low molecular weight and increased if the molecular weight of the added polystyrene was large. In the blends containing low molecular weight PS, the area per junction increased with the addition of PS, while it kept almost constant for the blends containing high molecular weight PS. In the fixed PS concentration,increasing the homopolymer molecular weight increased the size of PS and PB microdomains and decreased the area per junction. The swelling in the PS microdomains became dramatically asymmetric with an increase in the homopolymer concentration and/or the homopolymer molecular weight.Over the cylinder-lamellar transition induced by the addition of thepolystyrene,the lateral expansion increased dramatically.