The deformation, breakup and morphology relaxation of a commercially important polycarbonate (PC)/poly(styrene-co-acrylonitrile) (SAN) blend under and after steady shear flow have been studied in situ by combined phase contrast optical microscopy (PCOM), small angle light scattering (SALS) and rheometry. Under steady shear flow, the morphology of PC/SAN blends evolves via repeated deformation, breakup and finally string-like morphology. The data can be qualitatively interpreted with the mode-coupling renormalization group (MCRG) model. At high shear rate, shear-induced mixing effect is found to be saturated and no further shear-induced homogenization can be observed, which may be due to the fact that the experimental conditions (T, C) is far from the critical region and the shear suppression of concentration fluctuations is limited. Upon cessation of shear, different relaxation mechanisms are found. For low shear rate, the anisotropic ellipsoids retract to isotropic domains after shear cessation; while for higher shear rate, the string-like pattern breaks into necklace-like structure first and then the aligned structure relaxes to an isotropic distribution via diffusion process. The slow coarsening process upon cessation of shear is attributed to high viscosity of PC matrix and viscoelastic effects. (C) 2012 Elsevier Ltd. All rights reserved.