Camphor-10-sulfonic acid (HCSA) doped polyaniline (PANI)/poly(ethylene oxide) (PEO) composite nanofibers with different compositions (12 to 52wt.% of PANI) were synthesized by an electrospinning method and their properties including optical, electrical and sensing were systematically investigated. FT-IR shows that an increase of IR absorbance ratios of aromatic CC stretching vibration of benzenoid rings of PANI to COC symmetric vibrational modes of PEO confirmed that the PANI content in nanofiber mats increased proportionally with increase in PANI content in electrospinning solution. The band gap of PANI was determined to be 2.5eV using UV-Vis spectroscopy. The electrical conductivities of the nanofibers increased with an increase in the PANI content in the nanofibers. Additionally, the sensitivity toward NH3 increased as the PANI content increased, but branched nanofibers reduced sensing response. The humidity sensitivity changed from positive to negative as the PANI content increased. The electron transport mechanism was studied by measuring the temperature dependence electrical resistivity. The negative temperature coefficient of resistance revealed a semiconducting behavior for the PANI/PEO nanofibers. The activation energy, calculated by Arrhenius plot, increased as the PANI content decreased. The power law indicated that electrons were being transported in a three dimensional matrix, and the longer hopping distance required more hopping energy for electron transport.