In this work, Si-doped cross linked carbon nanofibers (Si/CNF) were prepared by electrospinning poly-acrylonitrile (PAN) solutions containing polyhedral oligomeric silsesquioxane (POSS) as Si source, followed by thermal treatments. Scanning electron microscope (SEM) observations showed the smooth surfaces of pure PAN nanofibers, while PAN/POSS nanofibers with obviously rough or porous surfaces were obtained with relatively high POSS content. X-ray diffraction (XRD) identified the presence of POSS in the composite nanofibers. After thermal treatments, cross linked Si/CNFs with various morphologies were obtained. Energy-dispersive X-ray (EDX) and X-ray photoelectron spectroscopy (XPS) analyses confirmed the successful incorporation of Si into CNFs. Raman spectra suggested the degree of carbon ordering of Si/CNFs reduced slightly when compared with pure CNF. The obtained nanofibers exhibited typical performances of electric double-layer capacitors (EDLC). The specific capacitance of Si/CNF showed a moderate increase with POSS content and reached a maximum of 175 F/g at 1 A/g with PAN:POSS equal to 100:5. Meanwhile Si/CNFs exhibited excellent cycling stability after 1000 cycles. The influence of POSS on the capacitive performance might be attributed to the combined effects of evolution of morphology, conductivity and surface properties of Si/CNFs.