Abstract: A damage identification index should be sensitive to the local structural damage and its computational response should cater for the requirement of on-line structural monitoring system. By adopting natural excitation technique and mathematical statistics theory, a damage index is established, which is based on the strain measurement of structures under environmental excitation. Numerical simulation is carried out, it reveals that the damage index based on strain is more sensitive than that based on displacement or natural frequencies. The damage index can detect the occurrence and find out the approximate location of structural damages.