Carbonization followed by calcination in air has been developed to synthesize porous Sn-O2 based nanofibers. The precursor nanofibers were synthesized through electrospinning. The optimal synthesized condition was investigated according to the gas sensing properties of pure SnO2 nanofibers through adjusting the heating rate in air. Gas sensors based on 1% Pd doped SnO2 nanofibers under this optimal synthesized condition were fabricated, and the gas sensing properties was systematically investigated. The experimental results indicate that all the carbonized nanofibers have a porous microstructure. For the pure SnO2 nanofibers, carbonized nanofibers also have a hollow structure. At the optimal synthesized condition, the sensor based on the carbonized SnO2 nanofibers Exhibits 2.6 times higher response (20.4) than pristine pure SnO2 nanofibers (7.7) toward 100 ppm ethanol. However, the selectivity is almost unchanged. For the Pd doped SnO2 nanofibers, the gas response is improved from 10 to 24.6 to 100 ppm toluene at optimum operation temperature under the carbonization process. The sensors also exhibit a low detecting limit (1.6-500ppb toluene) and a short response time (similar to 3s). The evolution process and the formation mechanism were also discussed.