The effect of compatibility on the improvement of the toughness of polyphenylenesulfide (PPS) blended with styrene-ethylene-butylenestyrene block copolymers (SEBS) has been studied. In PPS/SEBS polymer alloy having SEBS, which was highly modified by maleic anhydride, it was confirmed that the elastomers were dispersed finely because of high compatibility with the PPS matrix, and the toughness was improved efficiently by the release of constraint of the strain due to void formation in the elastomer particles. On the contrary, in the PPS/SEBS polymer alloy having the conventional SEBS or SEBS, which was lightly modified by maleic anhydride, the diameter of dispersed elastomer particles was larger, and the particles were deformed into a rod-like shape, and oriented along to the direction of injection flow, particularly in high content of elastomer. These oriented particles were fractured easily, and many cracks, which were distributed along to the direction of flow were formed. These cracks would result in suppression of improvement or decrease in toughness of the polymer alloy. (C) 2000 Elsevier Science Ltd. All rights reserved.