The changes on vibration properties were investigated to develop a NDT for FRP in working conditions. Glass/vinylester fabric composites were used for a test specimen and a damage specimen was made by 1-cycle load-unload static tensile tests. Impulse response tests, which are very useful to carry out in working conditions, were performed before and after the static tensile tests in order to estimate changes of the vibration properties of the specimen. It became clear from the comparison of vibration properties before and after the test that vibration properties were changed by the damage (i.e. the decrease of natural frequencies and gains at resonance, and increase of damping ratios). Also, it became clear that the changes of vibration properties clearly occurred with the increase of damage. It was remarked that there was a linear relationship between the changing rate of vibration properties and the increasing rate of transverse cracks. These results showed that there was able to estimate the value of damage from vibration analysis.