Multi-walled carbon nanotube (MWCNT) reinforced alumina composites were prepared by spark plasma sintering using pristine MWCNTs and acid-treated MWCNTs. The effect of acid treatment on the structure and surface potential of the MWCNTs was examined by transmission electron microscopy (TEM) and zeta potential analyzer. It is demonstrated that with the acid treatment of the MWCNTs, we have deliberately introduced nanoscale defects and negatively charged functional groups on the surface of the MWCNTs. The average depths of the defects are typically 4.8-10.8 nm. Mechanical measurements revealed that surface modification of the MWCNTs is effective in improvement of bending strength and fracture toughness of the MWCNT/alumina composites. Only 0.9 vol.% acid-treated MWCNT addition results in 27% and 25% simultaneous increases in bending strength (689.6 MPa) and fracture toughness (5.90 MPa m1/2), respectively.