Al_2O_3 and 3% (mole fraction) yttrium tetragonal zirconia was joined together by a process of directly joining their green body in plastic state and following co-sintering. Joining strength, joint microstructure, stress state of joining interface, and diffusion between joined compositions were studied. Experimental results show that a transition interlayer containing both joined particles is formed in the joint which is dense and pore free, fracture taken place at Al_2O_3 matrix near the joining interface but the joining strength is lower than normal Al_2O_3 matrix because of the complex stress state near the interface. Apart from the effect of plasticity of green bulk on joining before sintering, it is postulated from the HRTEM/EDS studied results that the diffusion of Al 3+ to the ZrO_2 grain during the sintering process contributed to the interface bonding which led a success joining of Al_2O_3 and ZrO_2 composites. Al 2 O 3 and 3% (mole fraction) yttrium tetragonal zirconia was joined together by a process of directly joining their green body in plastic state and following co-sintering. Joining strength, joint microstructure, stress state of joining interface, and diffusion between joined compositions were studied. Experimental results show that a transition interlayer containing both joined particles is formed in the joint which is dense and pore free, fracture taken place at Al_2O_3 matrix near the joining interface but the joining strength is lower than normal Al_2O_3 matrix because of complex stress state from the interface. Apart from the effect of plasticity of green bulk on joining before sintering, it is postulated from the HRTEM / EDS studied results that the diffusion of Al 3+ to the ZrO 2 grain during the sintering process contributed to the interface bonding which led a success joining of Al 2 O 3 and ZrO 2 composites.