Multiwalled carbon nanotubes (MWNTs) were oxidized by nitric acid in order to simultaneously induce defects and hydrophilic (carboxyl) groups on the surface of MWNTs, on the basis of the concept that defects and carboxyl groups could contribute to the increase in the number of adsorption sites of gas molecules such as NO2. It was observed by transmission electron microscope (TEM) that oxidized MWNTs had a rough sidewall surface. The sensor response to dilute NO2 (Ra/Rg) of an oxidized MWNT-based sensor was 50% higher than that of an as-grown MWNT-based sensor, suggesting an increase in the number of NO2 adsorption sites with an increase in the density of defects on the surface of MWNTs. On the other hand, the p-n junction of a semiconductor generally contributes to increasing electric resistance by forming a space charge layer. A space charge layer could be expected to enhance NO2 gas adsorption. Thus, MWNTs as p-type semiconductors were modified with an n-type semiconductor such as WO3 and SnO2 nanoparticles, which generally exhibit an excellent sensor response to dilute NO2. MWNT-WO3 and MWNT-SnO2 composite sensors showed a fairly good sensor response (Ra/Rg) to dilute NO2, compared with that of the MWNT-based sensor. The addition of an n-type oxide to MWNTs contributes to the formation of a large depletion layer within MWNTs by the p-n junction on contact points between MWNTs and oxides, resulting in an increase in resistance in air and the enhancement of NO2 adsorption.