By using the direct ratio between the minus exponential of every gray value of two-dimensional X-ray image and the thickness of the slice, the attenuation coefficient of the apple was calculated on the condition of the invariable experimental parameters. The mathematic model to predict apple cubage was established, and the cubage of the apple was predicted. The result of the experiment testified the exponential relation between the attenuation of the X-ray energy and the thickness of the apple slice, the regression correlation coefficients for calibration is 0.9932, and the average attenuation coefficient is 0.0114. The regression model was obtained with the correlation coefficient for calibration and prediction coefficient of 0.9809 and 0.9203, respectively between the actual value and predicted value of apple cubage. The mathematic model could calculate the cubage of the apple reliably and credibly.