How to uniformly disperse carbon nanotubes (CNTs) and densely sinter green body are the key issues to prepare carbon nanotubes reinforced alumina (Al2O3) composite. We prepare CNTs-Al2O3 powder by shearing extruding dispersion method, and then obtain CNTs-Al2O3 composite by hydrogen atmosphere pressureless sintering. 93% of the relative increased fracture toughness can be achieved, when the CNTs content of composite is 1%. Because of Absorbing energy by pulling CNTs out from alumina matrix and refining grain by CNTs bundles sitting along alumina grain, the fracture toughness is improved obviously. By XRD and SEM analysis, the residual tensile stress which is caused by embedding CNTs into alumina matrix also plays an important role for the increase of fracture toughness.