Since an aqueous two-phase polymer solution (ATPS) contains mainly water, it provides a mild environment to the biological materials; and therefore, the extraction by means of ATPS is widely used in the field of separation of biologically active materials. In this work, the liquid-liquid equilibrium (LLE) of ATPS containing polyethylene glycol (PEG) and dextran (Dex) were experimentally investigated. In this effort, the gel permeation chromatography (GPC) was used for analysis of the composition of the equilibrium phases. It was found that the larger molecular weights of the phase-forming polymers lead to the wider phase-splitting region, and the larger difference between the molecular weights of the polymers resulted in the more asymmetric binodal curve. Furthermore, a clear polymer fractionation on the phase separation was observed from the results; the MWDs of each polymer in the coexisting phases were different from each other as well as from the original distribution. While the average molecular weight of Dex in the top phase decreased with the increase of the tie-line length (TLL), that of the bottom phase asymptotically approached that of the original. In addition, the continuous dilution method was found to be a rapid and accurate method for complete determination of the phase boundary.