The applicability of structural monitoring is discussed for crack-type damages of metal structures. Two types of monitoring methods are examined detection of existing fatigue crack and prediction of fatigue damage. As the detection of existing crack, the concept of health monitoring is employed. Five types of line sensors, i. e. conductive film-sensor, conductive-paint sensor, plastic optical-fiber sensor, glass optical-fiber sensor and carbon-fiber sensor are made on an experimental basis. The sensors are bonded on the surface of a welded joint model and two types of specimens by adhesives or painting, and fatigue tests are carried out. In these tests, the appearance that the fatigue cracks break the sensors is closely observed. Further, electric resistance of sensors or light strength output of laser beam are measured to perceive the instant of failure during the fatigue test. As the prediction of fatigue damage, method to attach sacrificial specimen on member is examined. A sacrificial specimen is designed such that magnified member stress is transmitted to the specimen and thus results in earlier crack initiation than the member. The sacrificial specimen is a center notched thin plate with 60 mm-length, 10 mm-width and 0.25mm-thick. The middle part in length of the specimen is coated by Teflon film and the whole of the specimen is sandwiched in between two thin epoxy resin plates. The sacrificial specimen is bonded on a smooth specimen by epoxy resin, and fatigue tests are carried out. From the test result, it is made clear that the sacrificial specimen developed in this study can fulfill the function to predict the fatigue life of structural members.