Polyethersulfone (PES) nanofibers are produced by electrospinning solutions of PES/dimethylforamide (DMF), PES/N-methylpyrrolidone (NMP) and PES/(NMP:DMF) (of different NMP:DMF ratios) at temperature of 40 degrees C and various levels of relative humidity (RH). The influence of environmental conditions on bead formation as well as surface and interior morphologies of electrospun fibers is discussed through the phase diagram of H2O/DMF/PES and H2O/NMP/PES systems. The former case has small miscibility area while the latter one has large of which. The results demonstrate the contribution of RH of operating environment to morphology evolution of nanofibers. If the size of miscibility area increases e.g. H2O/NMP/PES system, a higher values of RE is needed to stabilize the formation of fibers. For this system, low level of humidity leads to develop beads as well as bead-on-string morphology. Adding the second solvent i.e. DMF into the PES/NMP solution shifts the binodal boundary toward the polymer-solvent side meaning a smaller miscibility area. In consequence, formation of fiber can be stabilized under broad range of humidity levels i.e. from low to high level of humidity. Implications regarding formation of surface pores by manipulating phase behavior of ternary system as well as RH of ambient conditions are discussed related to physico-chemical nature of solvent.