In this study, the effect of relative humidity and temperature on the morphology of electrospun Poly(trimethylene terephthalate) (PTT) fibers is investigated. It is also examined the interaction between these ambient parameters and those of some important parameters such as charge density, tip to collector distance and flow rate. Dichloromethane (DCM) and trifluoroacetic acid (TFA) solvent mixture is used. Field emission scanning electron microscopy (FESEM) is used for determination of surface morphologies and average diameters of electrospun PTT fibers. The results show that average diameter of electrospun PTT fibers decreases with increasing of relative humidity and temperature values for all of the charge density, tip to collector distance and flow rate values. The highest rate of decrease is seen at 0.9 ml/h for increasing of RH from 30% to 70%. All of the obtained electrospun PTT fibers are circular shaped with smooth surface without bead formation for all RH and temperature values.