Pure and 1-3 mol% Co-doped WO3 nanofibers were fabricated via a feasible and simple electrospinning method and calcined at 500 degrees C. Co-doped WO3 nanofibers displayed mat-like porous network with coarse surface and diameter of approximately 100 nm. 3 % Co-doped WO3 nanofibers were well crystalized and composed of nanosized grains (10-20 nm). The phase structure transformed from orthorhombic to monoclinic, which was less symmetric, by doping with Co from 0 to 3 %. The acetone detection limit of 3 % Co-WO3 nanofiber sensor was 0.25 ppm, and its response to acetone was below 1.5 for healthy humans (<0.8 ppm) and above 2.0 for diabetes patient (>1.8 ppm), making it a promising candidate for human breath diagnosis of diabetes. The high response to acetone of 3 % Co-WO3 nanofibers was attributed to their porous structure and nano-sized crystal grains. The selective detection toward acetone against other disturbing gases was resulted from the lower symmetric monoclinic phase structure with more oxygen vacancies and structural defects on the surfaces.