We report the mechanical properties and microstructures of alumina ceramics reinforced with multi-walled carbon nanotubes (MWCNTs) that were surface treated with an acid composed of H2SO4 and HNO 3. The acid treatment time was in the range of 0-4 h. Analyses showed that the treated MWCNTs had negatively charged functional groups and nanoscale defects on their surface. In order to evaluate the effects of the acid treatment of the MWCNTs on the mechanical properties and microstructures of the composites, we investigated the dispersibility of MWCNTs and morphology on the fracture surface. We found that the acid treatment was useful to improve the dispersibility of the MWCNTs in the composites, even though their mechanical strength decreased due to the induced nanodefects. The experimental results revealed that the improved dispersibility of MWCNTs overcame their strength reduction and there was an optimal treatment time for enhancing the mechanical properties of the developed MWCNT/alumina composites.