An experimental study has been carired out to evaluate the residual mechanical properties of thermally damaged concrete using a nonlinear ultrasonic method. A nonlinearity parameter was measured by the impact-modulation technique, which sensitively reflects the degree of thermal damage in concrete. Key mechanical properties, compressive strength, elastic modulus, and peak strain were obtained from compressive stress-strain curves of cylindrical concrete samples. The correlations between the nonlinearity parameters and the mechanical properties were investigated for various conditions: high-temperature exposure time of 1, 2, and 3 hours and four different mixture proportions. The authors thereupon developed regression equations that can predict the residual mechanical properties from the measurement of a nonlinearity parameter, which indicates the degree of thermal damage.