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We present a micromachined NH3 chemiresistor sensor based on O2 plasma functionalized multiwall carbon nanotube (MWCNT)/ conducting polymer (CP) composites, and discuss its gas sensing mechanism. The FTIR spectra of plasma-treated MWCNTs indicate that oxygenated groups are created on the surface of CNTs after the plasma treatment. The plasma-functionalized MWCNT/PANI sensor exhibits a linear response of 3.34% per ppm NH3 for concentrations ranging from 10 to 100 ppm, and a sensitivity of about 7 times higher than that from a corresponding pristine MWCNT sensor. This dramatic change in resistance is attributed to the fact that the binding of the functional group with the defective CNT is stronger than that with the perfect one.