On acidic red soils, low phosphate availability is one of the major limitations to crop production. Mobilization of insoluble phosphates by root exudates plays an important role in improving P nutrition of crops. In this study, mobilization of Al bound phosphate (AlP) by root exudates of soybean was systematically studied using anionic, cationic exchange resins, and molecular membrane of different molecular weight. Mobilization experiment indicated that AlP mobilization by anionic fraction of root exudates was significantly higher than those by cationic or neutral fraction. Among above 8K, 8~3.5K, 3.5~1K, and below 1K fraction, root exudates of below 1K fraction has the largest ability to mobilize AlP in comparison to other fractions of root exudates. Mobilized AlP by below 1K anionic fraction were 13.2 mg/pot and 9.3 mg/pot for P-deficient and P-sufficient treatment respectively, which represented 71% and 57% of the AlP mobilized by total root exudates. Analysis of root exudates of below 1K anionic fraction by ion chromatography indicated that a small amount of citrate and malate was exuded by both P-deficient and P-sufficient soybean roots, which took up 10%~20% of organic acid triggered by Al treatment. However, no difference of organic acid exudation was observed between P-deficient and P-sufficient soybean roots. The above results indicated that other unknown compounds in root exudates of soybean might enhance the mobilization of AlP except for organic acids. Related search is deeply on going.