To investigate the correlation of viscosity–morphology–compatibility of polymer blends, polystyrene, polybutadiene, and styrene/butadiene triblock copolymer were selected as our materials. A capillary rheometer was used to measure the viscosity. All samples were blended by a two-roll mill, a Brabender, or by the solution casting method. The morphologies of extruded samples from the capillary were observed by SEM. The relation between the viscosity and the composition of the blend changes with shear rate. It was found that the better the compatability between the two phases of the blend the smaller is the disperse phase domain. The viscosity of polyblends is related to the compatability and composition of the two phase in this blend. In the present study, the rheological behavior of polyblends is explained from the viewpoints of morphology and compatability. © 1993 John Wiley & Sons, Inc.