In this paper, polyaniline/multiwall carbon nanotube (PANI/MWCNT) composites were fabricated through an in situ polymerization method and were applied for the detection of aromatic hydrocarbon vapors. The composites were characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy, and Raman spectroscopy. The SEM results showed that the PANI was uniformly coated on the MWCNT, and the coating thickness was dependent on the mass ratio of aniline monomer to MWCNT. The response to aromatic hydrocarbon vapors was investigated in several hundreds ppm ranges. The sensor showed an increase in conductivity, and the maximum response measured at 1000 ppm was several tens of percent.