This study was conducted to establish a non-destructive inspection method based on ultrasonic echo measurement. The method was mainly aimed to estimate the damage condition and remaining strength of glass-fiber reinforced plastic (GFRP). After immersion in hot water for 1 day-1 month (86.4 ks-2.42 Ms), the damage condition of GFRP was optically observed in detail and ultrasonic echo was measured over the surface. Further, the changes in the mass, Barcol hardness and bending strength were systematically investigated. Isolated delamination of glass-fiber cloths was found at the immersion period of 14 days. Then, its size and opening distance were rapidly increased. The results showed that such progress of delamination can be evaluated by the decrease in the ultrasonic echo parameter (V value) which was proposed in this study. The surface distribution and frequency distribution of V value gave more useful data to evaluate the damage condition. Furthermore, it was shown that the remaining strength of GFRP can be evaluated using V value as a damage parameter.
