Solid state bonding of Si3N4 and Ni is considered to be a simple ceramic/metal diffusion bonding as the case of Al2O3/Nb. There are two kinds of bonding interfaces observed by transmission electron microscope(TEM). In addition to the directly bonded interface, an amorphous-like phase was found in some regions of the Si3N4/Ni interface. It is responsible for the degradation of bonding strength. In view of the experimental results, the existence of the amorphous-like phase in bonding interface results from the formation of SiO2 and accumulation of Al2O3 and Y2O3, which were used as sinterfng binders of Si3N4, via intergranular diffusion during the bonding process.