The effect of simple shear flow on the phase behavior of blends in styrene-acrylonitrile random copolymer with 29.5 wt % acrylonitrile content (SAN-29.5) and poly(methyl methacrylate) (PMMA) of different molecular weights ranging from 7000 to 396 000 g/mol has been investigated as functions of shear rate, sample composition, molecular weight, and rotation speeds of parallel plate by using a shear apparatus. Only shear-induced mixing was observed for all of the measured samples, and the shear effect was found to be composition and molecular weight dependent. The shear range was also extended by using different rotation speeds of plate ranging from 0.5 to 5.0 rad/s under constant sample thickness, and at first the cloud points increase monotonically with shear rate and then become almost constant regardless of the applied shear rate values. The normalized shift of cloud point Delta T((gamma) over dot)/T(0) = {T((gamma) over dot) -T(0)}/T(0) depended on the molecular weight of PMMA remarkably. From the measurement of the complex dynamic viscosity (eta*), the viscosity ratio of PMMA/SAN blends (eta(PMMA)/eta(SAN)) was estimated, and it was shown that the molecular weight dependence of the normalized shift of cloud point was due to the values of viscosity ratio between pure polymers. A maximum elevation in the normalized shift of the cloud points took place when the viscosity ratio is close to unity.