Abstract: The fatigue tests were carried out to a 304 stainless steel plate-fin structure,and the fatigue fracture mechanism was analyzed.The crack is initiated at the top fillet,and then it propagates to the fin until failure.During the brazing,some brittle phases are generated in the fillet where the stresses are concentrated,which result in the initiation of crack at the fillet.The fatigue crack propagation is composed of four stages.The first stage is the crack initiation around the brittle phases at the fillet under the cyclic loading.The second stage is the crack propagation in the fillet metal.The third stage is that the crack crosses the interface between filler metal and solid solution and then propagates along the solid solution.The fourth stage is that the crack crosses the interface between solid solution and fin,and then it propagates along the fin until the failure.