Carbon nanotubes (CNTs)-reinforced Al2O3-matrix nanocomposites were fabricated by a serious of physical dispersion methods and subsequent spark plasma sintering. Crystalline structure, density, mechanical properties and electrical conductivity of the composites were evaluated, and the strengthening and toughening mechanisms were discussed. The results show that there exist angular drifts of the X-ray diffraction peaks for the Al2O3 matrix, which indicates the presence of residual compressive stress in the matrix. The bending strength increased initially with the increase of CNTs content, and reached the maximum with 1.0 wt% CNTs content. The fracture toughness of the nanocomposites also increased with the addition of CNTs, and the composite with 0.5 wt% CNTs has the maximum. The resistivity of composites decreased significantly with the increase in CNTs content, decreasing by seven orders of magnitude when the CNTs content is 2.0 wt%. According to the observations of crack propagation path and fractographs, four toughening mechanisms are summarized: CNTs pull-out, grain interface bridging, crack deflection and crack bridging.