A kind of strain sensor using fiber Bragg grating (FBG) was developed t o study the problem of determining whether concrete structures crack or not. The characteristics of this kind of strain sensors are studied. This kind of strain sensors are used to monitor reinforced concrete frameshear wall structure mod e ls in destructive experiments on a shaking table. It turns out that it can be de termined whether the structure cracks or not according to the monitored data and the results are in good agreement with what are observed at the testing site. T herefore, it is concluded that the developed FBG strain sensor is able to direct ly and accurately monitor the local strain variation on reinforcing steel bar in the structure and then the tension strain in concrete on member surface can be obtained by derivation; it can be accurately determined whether the structure cr acks or not by comparing the obtained tension strain with the ultimate tension s train of concrete.