Visual discomfort when looking at stereoscopic three-dimensional (3D) displays has been described as the number one health issue for 3D industry. This paper focuses on the assessment of visual comfort in 3D contents and proposes an objective model to assess the visual comfort induced by possible factors. Based on the fact that visual comfort is a subjective sensation that accompanies the changes of 3D characteristics, a general model with a form of power series of logarithm, was presented to predict the level of visual comfort for candidate factors based on Weber-Fechner's Law. In particular, a psychophysical experiment was conducted to subjectively measure the degree of visual comfort induced by disparity factor in motion 3D contents. The results of experiment show that visual comfort decreases as the disparity increases on both crossed and uncrossed directions, and the comfort disparity is in the range of -120 to 115 minute of arc. Then the proposed model was fitted based on psychophysical experiment data to find the optimal value of coefficients. The coefficients of determinations (R<sup>2</sup>) were examined to investigate the goodness-of-fit of the model for different powers, and the Weber-Fechner's form of the model was discussed.