The contact stiffness of a glass fiber-reinforced plastic, EL-850, has been measured at room temperature and in liquid nitrogen. Dimensions of the specimens were 20×20mm² with three different thicknesses of 2.7, 4 or 8mm. Contact pressures up to 80MPa were applied to the specimens. Results showed how the contact stiffness increased with the applied contact pressure. It seemed that roughness and waviness on the surfaces of the EL-850 were factors which reduced contact stiffness. A computer simulation of contact stiffness, which took account of only the waviness, and measurements of the real contact areas using thin PET films, indicated that the waviness considerably affected contact stiffness. Contact stiffness measured in liquid nitrogen was less than that measured at room temperature when the contact pressure was low. This result was presumably caused by greater waviness of the specimen because of thermal distortion caused by liquid nitrogen.