The fabrication of core/shell structured ultrafine nanofibers is an important challenge in nanomaterials because of their many potential future applications. We succeeded in fabricating novel core/shell-like nanofibers by a phase-separation process during the electrospinning of simple blend solutions containing various ratios of polyacrylonitrile (PAN) and fluorinated polyimide (PI). In addition, it was revealed that certain blend ratios produced branch structured nanofibers while maintaining the core/shell-like structures. Selective extraction of PI by acetone immersion and characterization by scanning electron microscope, Fourier transform infrared and X-ray photoelectron spectroscopy clearly demonstrated that the shell layer and the branched moieties were composed of PI, whereas the core domain was mainly formed by PAN. The process enabled control over the thicknesses of the core and shell domains as well as the diameter of the branched moieties derived from the main nanofibers by changing the molecular weight of PI. Interestingly, the water contact angle of nanofibrous membranes composed of PI/PAN = 80/20 (wt/wt) was 144 degrees, indicating strong hydrophobicity of the nanofibers. These results demonstrated that the core/shell structured branched nanofibers can control the surface properties and have greater potential for the development of highly functional nanomaterials.