The tests were conducted on three types of glass reinforced plastic specimens namely laminates with random mat, woven roving mat and unidirectional mat. In all the above types of specimens, it is found that the natural frequencies decrease by about 9% for the 80% damaged state. The increase in damping ratio is much higher, as high as 98% for the same state of damage. the decrease in Young's modulus in the longitudinal direction is of the order of 20%. Changes in mode shapes are barely perceptible except for localized damages. Measurements of these changes in vibration responses show promise as a means of non-destructively testing the structural integrity of fiber-reinforced composite structural members.