The glass fiber reinforced plastic (GFRP) with the corrosion resistance properties has been used in water environment as a structure material. Now, a non-destructive method is required in order to verify the decrease of mechanical properties during using GFRP. An ultrasonic detecting method was chosen from some non-destructive methods in the study. Eight kinds of UD-GFRP, which the immersing time was changed in 80±3℃ distilled water, were prepared. The tensile test was carried out to estimate the change of mechanical properties under water environmental degradation. As a result, it become clear that interfacial debonding causing the fall of tensile strength increased and the interfacial debonding became a factor of the source of reflection of an ultrasonic wave.