This study was undertaken with the intent of examining the aqueous polymer two-phase system for microscale extraction of protein. Polyethylene glycol (PEG) solution including protein and dextran solution without protein were separately poured into two inlets of a microprocessing device with a microchannel (500 mum width, 300 mum depth). The concentrations of the two polymers were 90 g/L and that of BSA was 200 mug/mL. Protein concentrations in the PEG and dextran layers were measured, and the partition coefficient was calculated as the ratio of the protein concentrations in the two layers. The partition coefficient of the microprocess found to be about 0.2, equal to that obtained by conventional centrifugal separator. As control experiments, (i) two-phase extraction without mixing, and (ii) aqueous polymer two-phase extraction using Teflon tubes (phi = 2 mm or 1 mm) were carried out. As a consequence of increase in the specific surface area, the extraction rate in the microprocess was improved, and the extraction period was reduced below that of macro scale processes. In addition, a kinetic model of protein extraction in microsize aqueous polymer two-phase systems was constructed, and the overall mass transfer coefficient K of the microdevice was calculated. It was observed that an increase in the specific surface area resulted in an increase in K. We also confirmed that volumetric mass transfer coefficient could be evaluated from the equivalent hydraulic diameter and linear velocity.