The water activities in aqueous mixtures of polyethylene glycol (PEG) and hydroxypropyl starch (PES) with different molecular weights, both binary and ternary systems, were measured by using the improved isopiestic method. The modified Fitter's model was used to calculate Liquid-liquid equilibrium (LLE) in PEG/PES aqueous two-phase systems. The model parameters were evaluated directly from the experimental water activities. Then the phase diagrams of LLE of PEG/PES aqueous two-phase systems with different molecular weights were successfully predicted using only one group of model parameters. Good agreement with the experimental data of tie line was obtained, with most of average absolute deviations being less than 1.5 wt.%.