By electrospinning method, fibers are produced with diameters ranging from hundred nanometers up to a few micrometers. In this study, the influence of ambient temperature and relative humidity on the morphology of electrospun polyetherimide (PEI) fibers at varying electrospinning parameters such as solution concentration, applied voltage, and tip-to-collector distance was investigated. 1-methyl-2-pyrrolidinone was used as a solvent for PEI. Morphology and diameter of electrospun PEI fibers were examined by field-emission scanning electron microscopy. It was seen that 20 wt% of the PEI solution shows the most increase in average diameters from 30 to 70% RH at 15, 25, and 35 degrees C. It was also seen that 20 wt% of the PEI solution shows the most decrease in average diameters from 15 to 35 degrees C at each of the RH values. An interesting result was observed for 12 cm of tip-to-collector distance. The average fiber diameters firstly decreased from 15 to 25 degrees C, and then increased from 25 to 35 degrees C for all of the RH values.