In recent years, many researchers have been studied and developed the nanofibers which have a large specific surface area and a small pore size. The polymer nanofibers are being used or finding uses in filtration, biomedical applications including wound dressings as structural elements in artificial organs, and reinforced composites. Thus these special needs have stimulated recent studies and renewed interest in the process, quantitative technical and scientific information regarding process and product characterization are extremely limited. The electrospinning can allow producing and encouraging the utilization of nanofibers. The surface morphology of e-spun fiber is affected by many variables that involve polymer concentration, applied voltage, spinning distance, air friction, gravity and ambient parameters. This study shows the effect of relative humidity on the microstructures of the e-spun Polystyrene (PS) fiber.A PS solution is prepared from PS pellet (Aldrich, molecular weight 170,000 g/mol) which is dissolved with Tetrahydrofuran and Dimethylformamide (Junsei Co.) in the ultrasonicator. The concentration of the PS solution was 7 weight%. The Spinning occurs from the droplet of solution protruding from the 0.7 mm internal diameter of the tip. A positive electric potential is applied to the polymer blend solution by attaching the lead to the variable high voltage power supply (Chungpa EMT Inc.). It was directly connected to the copper wire located in the inside of the solution. A drum collector covered with aluminum foil is placed 10 cm vertically from the tip of the syringe as a grounded collector. The relative humidity was changed with four steps from 10 to 70%. All experiments were performed at the constant temperature-humidity chamber. The surface morphology and the diameter of the e-spun fiber were investigated by the Scanning Electron Microscope (AMRAY model 1400), Atomic force Microscope (PSIA XE-100) and the image processor (GAIA Blue).The relative humidity has shown a great influence on the fiber diameter in terms of the analysis of SEM and measurement of fiber diameter which are given in FIG. 1 and TABLE 1. When the relative humidity was 10%, e-spun fibers have the average diameter of 130 nm (FIG. 1-a), but as the relative humidity increased to 70%, the fiber diameter gradually thickened to 380 nm (FIG. 1-d). The higher relative humidity makes thicker fiber because the higher electrostatic charge on the fiber surface has the opportunity to split the fibers more. Thus it results in producing thinner fibers in the travel of the fibers towards the target. In other words, the higher moisture can produce thicker fiber due to more discharge of electrostatics on the surface of the polymer solution.