To investigate the effects of Mo addition on the microstructures and mechanical properties of Al_2O_3$ ceramics, two kinds of Mo particles with average sizes of 2-${mu}{ extrm}{m}$ and 6-${mu}{ extrm}{m}$ were used as additives. It was shown that Mo particles inhibited the grain growth of Al_2O_3$, and the smaller Mo particles were more effective. In case of 2-${mu}{ extrm}{m}$ Mo dispersion, the bending strength and the fracture toughness were increased. Dispersion of 6-${mu}{ extrm}{m}$ MO did not increase the strength but improved the fracture toughness a little. The toughening mechanisms of Al_2O_3$-Mo composites are thought to be the crack deflection and microcracking mechanisms.