In this paper, a colloidal processing route was adopted to disperse multi-walled carbon nanotubes (MWNTs) into alumina powders homogenously. Alumina matrix composites containing 0.1 to 2 weight percent of MWNTs were successfully fabricated by pressureless sintering. Also, 1wt% MWNTs-alumina composites were prepared by hot pressing for comparison. It was found that when the sample was sintered at 1450oC, the addition of 1wt% carbon nanotubes led to 10% increase in bending strength compared with monolithic alumina. The reinforcement mechanism was discussed based on the microstructure investigation. The broken nanotubes and pullout of MWNTs at interfaces are efficient in transferring the load from the alumina matrix to the nanotubes, leading to the improvement of the mechanical properties.