A direct approach for fabricating nanoporous polymer fibers via electrospinning has been demonstrated. Polystyrene (PS) fibers with micro- and nanoporous structures both in the core and/or on the fiber surfaces were electrospun in a single process by varying solvent compositions and solution concentrations of the PS solutions. The porous structures of the fibrous mats were characterized by field emission scanning electron microscopy and Brunauer-Emmett-Teller measurements to confirm that they could be accurately controlled by tuning vapor pressure of tetrahydrofuran (THF) and N,N-dimethylformamide (DMF) solvent mixtures and PS concentrations in the solutions. As the solution concentration decreased, the average fiber diameter decreased, whereas the bead density increased dramatically to show a beads-on-string morphology. Both the specific surface area and pore volume of the fibrous mats showed a unimodal distributions centered at 1/3 THF /DMF mix ratio. Fibers formed from 5 wt % PS in the 1/3 THF and DMF mixtures had the largest specific surface area of 54.92 m(2) g(-1) and a pore volume of 0.318 cm(3)g(-1), respectively.