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The effects of various surface modifications on the gas sensing properties of multiwalled carbon nanotube (MWCNT)/polyaniline(PANI) composite films are investigated and compared. The functionalization of MWCNTs is carried out by using different chemical and plasma treatment methods. Four materials are studied: chemically functionalized multiwalled carbon nanotubes(c-MWCNTs), O2 plasma functionalized multiwalled carbon nanotubes(p-MWCNTs), c-MWCNT/PANI films, and p-MWCNT/PANI films. We find that the resistance of the p-MWCNT/PANI films increases by about 17% from 10 to 100 ppm NH3 gas, and the maximum sensitivity to NH3 gas is about 2 times higher than that from a c-MWCNT/PANI sensor. The linearity of the response to NH3 gas of the devices using p-MWCNT/PANI film as the sensing element is much better than that of the devices using c-MWCNT and c-MWCNT/PANI films. These results indicate that the plasma treatment is more effective modification process that leads to the improvement in the performance of MWCNT/PANI gas sensors.