In this study Glass fiber reinforced plastics (GFRP) was experimentally investigated by ultrasonic method. Test specimen is cut off from the sheet with angle from the glass fiber direction. At first, velocities of transverse waves propagating through the thickness direction are measured by rotating the transducer. Two polarization direction are along the fiber direction and perpendicular to it. Travelling velocity with the polarized direction to fiber is faster than that of perpendicular to it. Acoustical an isotropy of GFRP is about 2.2% which is larger than that of steel. Velocity change due to the applied stress are studied in uniaxial tensile test. It changes linearly with the applied stress and from the result the acoustoelastic constants are calculated.

For the evaluation of the development of an isotropy, we applied the strain e of 0, 0.25, 0.5% ... to the specimen under the uniaxial tensile test. Velocity measurement are carried out in the each stage of strain. In the specimen with angle = 0, fast and slow velocity difference V₁ - V₂ decrease linearly with applied strain. It means that elastic an isotropy of GFRP develops lineally with the strain in a small range deformation.