The mismatch of coefficient of thermal expansion (CTE) between AIN ceramic and metal is one of the main challenges to join AIN ceramic to metal successfully. Joining of AIN ceramics to metals was investigated by using diffusing bonding with various surface modification interlayer to satisfy its bonding challenges in this paper. Radio frequency (r.f.) sputtering was first applied to deposited titanium film on AIN surface to improve its wetting. Plasma spraying and brush plating were utilized to obtain FGM interlayer to reduce residual stresses caused by mismatch of CTE in AIN-to-Cu joint. On the basis of study of the effects of joining process conditions on the mechanical strength of joint as well as microstructures and residual stresses at the interfaces of AIN/interlayer/Cu, it can be deduced that the joinability of AIN ceramics to metals is in directly proportion to the values of the diffusion coefficients of metal atoms inside AIN, but in inversely proportion to the mismatch values of CTE between AIN and metals, and both of the values could be improved by using proper FGM interlayer.