Abstract: In this paper, the statistical properties of instantaneous distoition of which the distribution of APD (amplitude probability density) of the fluctuating pressure is Gaussian are described, and the mathematical lelations between those propeiites, the steady-state distortion and the tuibulence are given. It shows that the APD of the instantaneous distortion is not always Gaussian and is determined only by the steady-state distortion index and the distortion of turbulence. The maximum value of Non-Gaussian instantaneous distortion can be estimated by the method proposed in a paper by present authors with the effects of the APD on the estimated maximum value considered. Comparison between the estimated results and the measured data by the authors shows that the accuracy of estimation of the maximum value can be improved while dispensing the assumption that the APD of instantaneous distortion is Gaussian.