The effect of device-downsizing upon the continuous extraction of bovine serum albumin (BSA) in an aqueous two-phase system was studied. BSA molecules in the aqueous solution of polyethylene glycol (PEG) were partitioned into that of dextran (Dex) through mu m- or mm-sized two-phase flow systems in circular capillary tubes. Mathematical model of BSA extraction kinetics was formulated with dimensionless forms. Four effects of device-downsizing were observed. First, downsizing enlarged the specific interfacial area between a PEG-rich solution and a Dex-rich solution, and in turn, elevated overall volumetric mass transfer rates of BSA in the PEG-rich phase and in the Dex-rich phase. Second, downsizing enlarged the proportion of the cross section of the PEG-rich solution, and in turn, elevated the space-time of the PEG-rich phase flow. Third, downsizing reduced the delay space-time of mass transfer of BSA into the Dex-rich solution. Finally, downsizing enlarged the range of overall volumetric mass transfer rates of BSA extraction. The diminution in size of two-phase flow separator towards micron-size was found to be effective to increase the level of overall volumetric mass transfer rates of BSA extraction.