摘要： With respect to harmonic generation measurements in isotropic materials, the amplitude of the third-harmonic wave generally depends on both β2 (the square of the second-order nonlinear parameter β) and γ (the third-order nonlinear parameter). Therefore, some care should be taken when measuring these parameters using the third-harmonic amplitude. The purpose of this paper is to investigate detailed theoretical and measurement techniques for the accurate determination of β2 which dominates the third-harmonic amplitude in most biological ?uids and crystalline solids. The theory deals with harmonic generation in materials with cubic nonlinearity and de?nes the formula for measuring β2 with corrections for diffraction and attenuation. These corrections are derived from the Westervelt equation and play an important role in the measurement of nonlinear parameters. The third-harmonic amplitude that varies with the distance in water is obtained for the measurement of β2. β is also measured from the second-harmonic amplitude for comparison. We also con?rm the required input voltage to stably generate the third harmonic and discuss the effects of diffraction and attenuation correction on the β2 determination. The measured β2 in the propagation range of 0.05–0.2 m agrees well with the square of the measured β, revealing that the third-harmonic amplitude is closely related to β2, not γ . This paper covers comprehensive theories, experimentation, and analysis related to the measurement of third-harmonic generation in isotropic media and can be immediately applied to crystalline solids.