The water and organic solvent extracts of commercial black bean (Glycine max (L.) Merrilx) and soybean (Glycine max (L.)) were determined their antioxidative capacity using low density lipoprotein (LDL) system. Measurement time was 0, 30, 60, 90, 120, 150, 180 and 240 min. The saturation line of malondialdehyde (MDA) production for healthy and atherosclerotic patients was approximately 85 and 105 nmole/mg LDL-C, respectively, and used as control. Since the lag phase show that the antioxidative capacity of commercial black bean and soybean was very close, it became very important to find a method to accurately distinguish from the slight difference between the two materials. The GM(1,1) model of grey system theory was applied to predicting four grey models on antioxidative effects of the water and organic solvent extracts of commercial black bean and soybean on healthy and atherosclerotic patient LDL. Thirty-minutes equal interval between 0 and 180 min was used to establish the grey model. After constructing the grey models, 3-minute equal interval was used to do grey prediction analysis. According to the characteristics of grey system theory, disappearing time of total antioxidative capacity in stead of traditional lag phase was used to compare the capacity and lasting of antioxidation. While the grey prediction disappearing time of water extract of black bean and soybean for the healthy were 474 and 450 min, respectively, the organic solvent extract were 321 and 309 min, respectively. While the grey prediction disappearing time of water extract of black bean and soybean for the atherosclerotic patients were 531 and 474 min, respectively, the organic solvent extract were 351 and 309 min, respectively. The above data show that in spite of the healthy or atherosclerotic patients, the water extract may have higher antioxidative capacity than organic solvent extract and black bean always better than soybean. In addition, despite water or organic solvent extract, black bean may have higher antioxidative capacity and last longer than soybean does. Black bean improves more in atherosclerotic patients than in the healthy. The grey prediction is better than traditional analysis and offers more accuracy.