High surface area polyaniline (HSA PANI) nanofibers were synthesized through oxidative polymerization of aniline in compressed CO2 using cobalt chloride as an additive. SEM and TEM analyses showed that the HSA PANI nanofibers had a coarse surface of very thin nanofibers. The HSA PANI nanofibers had a fairly uniform diameter range of 70-90 nm with a length of 0.5-1 μm, and showed an electrical conductivity (EC) of 3.46 S/cm. TGA analysis revealed that the HSA PANI nanofibers had more doping substances than did ordinary PANI nanofibers. In the case of the HSA PANI nanofibers, direct measurement of the surface area using gas adsorption method showed high value of 68.4m2/g, which was nearly twice that of ordinary PANI nanofibers. The HSA PANI nanofibers were used to fabricate the hydrogen sensor, exhibiting a large increase in resistance upon exposure to hydrogen gas. The hydrogen sensor in this work showed excellent characteristics, such as high sensitivity and short response time. The limit of detection (LOD) and limit of quantification (LOQ) of the hydrogen sensor were very low to show 40 ppm and 133 ppm of hydrogen, respectively.