The transient rheological behavior and morphology evolution of polymethyl methacrylate (PMMA)/polystyrene (PS) binary polymer blends with well-defined initial structure were measured in simple shear flow under isothermal conditions. The size and distribution of the dispersed phase and the composition of the blends were designed and fabricated by Computer Numerical Controlling (CNC) machining, photolithography, and micro-embossing. Compatibilizer can easily be placed at the interface of the two components during sample preparation. The effects of initial dispersed domain size, blend composition, and interfacial tension on rheological behavior and morphology evolution were investigated. It was found that the transient shear stress and first normal stress difference are very sensitive to these parameters. The transient rheological responses up to the breakup point are compared with those predicted by both Doi-Ohta and Vinckier-Moldenaers-Mewis models. (C) 2004 Elsevier Ltd. All rights reserved.