Rheological measurements have been an effective technique for the characterization of miscibility of polymer blends. This article investigates the viscoelastic behavior of poly (styrene) (PS) and poly (styrene-co-acrylonitrile) (PSAN) binary solutions in tetrahydrofuran (THF) relative to PS/PSAN/THF ternary solutions. The main emphasis was to correlate the miscibility of the blend and their rheological behavior. The shear viscosity and shear stress were investigated as a function of shear rate, and complex viscosity, loss and storage moduli as functions of oscillation frequency for various blend composition. The results were discussed in the light of miscibility criterion of polymer mixture. The present study revealed that only 50/50 composition has the values close to the additivity rule or intermediate to those of the neat polymers, thereby indicating very weak interactions between the blend components. On the basis of rheological results, it was concluded that the miscibility depends on the blend composition and shearing condition and that the blend under study was classified as almost immiscible. Keywords: Rheological properties; Miscibility; Polymer blend; Polystyrene (PS); Poly (styrene-co-acrylonitrile) (PSAN).