A microfluidic device for protein preconcentration based on the electrokinetic trapping principle is proposed. It comprises a Nafion strip which is simply cut from a commercially available membrane and integrated into a polydimethylsiloxane (PDMS) microfluidic structure using a guiding channel. By applying low voltage (<15 V), the device exhibits a preconcentration factor of >1000 of fluorescently labeled bovine serum albumin (BSA) within a few minutes. Moreover, a numerical study of the preconcentration effect is presented to microscopically understand the phenomenon in our type of device.