A new nonlinear acoustic modulation technique was developed to determine flaw locations in a material as a nondestructive evaluation method. Nonlinear interaction of two primary low-frequency continuous and high-frequency burst-type acoustic waves in the material can generate modulated signals. Such modulated signals show significant difference in the material with flaws. The experimental measurements were performed with steel and aluminum rods. Two PZT ceramic elements attached at the endsof rods were driven with two primary frequencies by separate functiongenerators through power amplifiers. Modulated signals detected at a receiving sensor were analyzed by a digital storage oscilloscope with the function of fast Fourier transformation. The second harmonic amplitude was observed 20 dB higher in a sample material with crack than that without crack. The crack location was also very accurately estimated by this nonlinear acoustic modulation technique at the sum and difference frequencies of the primary waves.