The specimens of Yttria Stabilized Zirconia(3Y-TZP) were bonded by a diffusion process. The microstructural evolution at the bonded interfaces was examined for the bonding conditions. We confirmed that above general sintering temperature(1500 °C), grain coarsening, which enhances the size fluctuation at the bonding interface, not only eliminates free surfaces but also is the main mechanism of bonding. In order to enhance the diffusion rate at the bonding interface, Ceria was added as a dopant, and grain boundary undulation was examined. We found that the bonding characteristics were improved by the Ceria doping. Specimens with an undulating bonded interface exhibited transgranular fracture, while specimens with a straight interface showed intergranular fracture. The undulation at the bonded interface is suggested to improve ceramic to ceramic bonding.