<,正,>, The results of this study reveals not only the sensitivity of the compatibility of PS with SBR to the molecular weight （MW） of PS and temperature, but also some other interesting characteristics, i. e., unusual morphology change during the process of mixing with increase in temperature of specimen preparation observed under optical microscope and double-peak UCST curves for three blends with PS of low MW from DSC data. Acording to the amount of inward shift of the component T,g,’s and the broadening of the transition regions, it may be said that this polymer pair is compatible only when the MW of PS is low, and even then there still exists micro-heterogeneity.