This study investigates the microstructure and mechanical properties of electrospun nanofibers from polyacrylonitrile (PAN)-dimethylformamide (DMF) solution at different relative humidity (RH) in the range from 14% to 60% and two different temperatures (20 degrees C and 40 degrees C). Nanofibers produced at low RH (22% or less at 20 degrees C) exhibit relatively smooth surface and solid core, whereas at higher RH (30% or higher at 20 degrees C) rough surface and porous core are observed. The resulting morphology is explained by means of H2O/DMF/PAN ternary phase diagram. At higher RH, the water diffusion into polymer-solution jet brings thermodynamic instability into the system leading to separation of polymer-rich phase and polymer-lean phase, where the later contributes to porosity. Higher process temperature (40 degrees C) yields larger miscibility area in the ternary phase diagram leading to formation of porous structure at relatively higher RH (40%). Tensile strength of nanofibrous yarns is found to vary from 80 MPa to 130 MPa depending on the processing temperature and RH. The amount of porosity is found to affect the tensile properties of nanofibers most significantly, although diameter and crystallinity play important role. Annealing is found to alleviate surface roughness and porosity and increase crystallinity. Tensile strength of nanofibrous yarns is found to improve by up to 25% after annealing. (C) 2017 Society of Plastics Engineers