Porous polystyrene (PS) fibers were prepared by relative humidity control during electrospinning process. The relative humidity and solvent mixing ratio strongly affect the surface morphology and average diameter of electrospun PS fibers. In the circumstance of 30% relative humidity at MC/EtOH (90/10, v/v), pores did not form on the surface of polystyrene fibers. However, as the relative humidity increased to 60%, pores appeared on the fiber surface at the same composition of solvent. In comparison, solvent ratio of MC/EtOH (80/20, v/v) gave rather smooth surface of PS fibers. When the MC/EtOH ratio are 90/10 (v/v) and 80/20 (v/v), electrospun PS fibers with minimum average diameter of 5,211 nm (SD = 1,986) and 5,315 nm (SD = 1,039) were prepared. Surface area and average pore size are found to be 30.7 m2/g and 8.7 nm, respectively, with the relative humidity of 40%.