In this research, the effects of MWCNT addition on mechanical properties of pressureless sintered MWCNT/alumina composites were investigated by using four types of MWCNTs having different nanostructures and fracture characteristics. Tensile-loading experiments revealed that there was an optimal annealing temperature for improving the fracture properties of MWCNTs. MWCNTs treated at 1800°C, 2200°C and 2600°C displayed enhancements to their nominal tensile strengths by factors of ∼5.3, ∼5.2 and ∼1.8, respectively, compared to MWCNTs treated at 1200°C. Fracture tests of MWCNT/alumina composites showed that the MWCNTs annealed at 2200°C was the most effective in improving the bending strength and fracture toughness of the MWCNT/alumina composites. These experimental results implied the effectiveness of the use of MWCNTs having a much higher load carrying capacity for preparation of tougher ceramic composites.