Abstract: There are two types of method for determination of polymer-polymer miscibility by viscometry. One is to measure the viscosity data of dilute solution for polymer blend, and the other is to measure the viscosity data of polymer 1 in a set of polymer 2 solutions. The polymer 2 plus small molecule solvent is treated as pseudo -solvent. Aoording to the viscosity theory of dilute solution for ternary system (polymer 1-polymer 2-solvent), the inherent relationship between two kinds of visoosity data is detailed. The thermodynamic parameter α criterion is modified using the weight additive law between the intrinsic viscosity of binary polyblend and the values of orresponding constituent polymer. Furthermore the modified criterion is verified by the experimental data published in literature. On addition, based on the viscosity data from the second measurement method, the miscibility of polyblends with various ompositions is alsopredicted.