Fatigue strength and fracture mechanism of ceramic/metal (Si3N4/Cu/SUS 304) joints were investigated under cyclic bending loading. Results showed clearly the characteristic of the cyclic fatigue, and the fatigue strength at l07 cycles was about one-half of the static bending strength. Different types of failure were observed for low-cycle and high-cycle fatigue regions. In the low-cycle fatigue region, the crack initiates at the copper interlayer near the interface of the ceramic side, while in the high cycle fatigue region the crack initiates throughout the entire copper interlayer. Fracture surfaces were examined by SEM and EPMA. Fatigue striations were clearly identified. 0n the basis of these results, a model is suggested for the fracture mechanism. The crack initiates and grows at the copper interlayer and finally kinks to the ceramic side. It is concluded that the copper interlayer dominates the fatigue strength of the joint.