Polyacrylonitrile (PAN) nanofibers with various morphological structures were produced by electrospinning by controlling the physicochemical properties (viscosity, surface tension, and electrical conductivity) in the precursor solution. Quantitative evaluation showed that fiber morphology was strongly influenced by precursor viscosity, and the viscosity thresholds affecting fiber morphology (ribbon-like, straight, spindle-like beaded, and beaded) were clarified. Using a monodisperse aerosol, the air filtration performance of filters made with these fibers was evaluated based on quality factors. Filters composed of beaded fiber showed the best quality in the air filtration test.