The purpose of this work was to investigate the dynamics of a graphite/epoxy composite material with delamination fracture under various environmental conditions. Modal testing is proposed as a nondestructive method to evaluate the effects of a delamination crack on the natural frequency and damping ratio. The effects of both thermal damage and moisture content on the graphite/ epoxy composite material were studied. The experimental impulse responses were compared with simulated values. The results show that the fundamental mode dominates the dynamic characteristics and that the delamination crack may decrease the natural frequency, but increase the damping ratio of the specimen. After the specimen was exposed to high temperatures, the stiffness and hence the natural frequency were increased. The damping ratio was decreased significantly after the specimen was subjected to prolonged exposure to a humid environment. From correlation between modal parameters and delamination cracks, it was found that the method of modal analysis was suitable for nondestructive evaluation of composite materials.