【Objective】Aluminum (Al) toxicity is an important factor that inhibits growth of plants in acid soils. Different plant species are often different in Al tolerance, and so are different varieties of the same species. Through a series of hydroponic experiments, this paper was to explore difference between two varieties of rice, Nipponbare and Kasalath in Al tolerance and mechanism responsible for the variation.【Method】 Plump seeds of the two varieties of rice were selected and placed in an incubator for germination at 25oC. Then seedlings similar in growing were divided into two groups, one undergoing treatment with 0.5 mmol L^-1 CaClsub>2 and pH 4.5 for 24 h (Treatment CK) and the other with 50 μmol L^-1 AlClsub>3 in addition to 0.5 mmol L^-1 CaClsub>2 and pH 4.5 for 24 h, too (Treatment Al). Root length was measured before and after the treatment with a ruler. After the treatments, some roots tips (0~1 cm) of the seedling were cut down with a knife, and placed into plastic tubes containing 1 ml 2 mol L^-1 HCl, separately, for 24 h extraction. Then the solutions were analyzed for Al concentration with ICP. Besides, some root tips were put direct into liquid nitrogen for refrigeration at -80 oC for late-on RNA extraction. To compare the two varieties of rice in Al-induced citrate secretion from the root system, some two-week-old seedlings were treated as in Treatment Al for 24 h. Then solution from the treatment was diverted into a column containing 5 g of cation resin (Amberlite IR-120B resin) for adsorption and then into another column containing 3 g of anion resin (AG 1 × 8 resin). The anion-resin column was then eluted with 2 mol L-1 HCl and the eluate condensed into solid through evaporation in a rotary evaporator. Then, 1 ml of milliQ water was used to dissolve the solid adhered to the evaporator for determination of content of organic acids with HPCL.【Result】It was found that under Al stress, Nipponbare was less inhibited than Kasalath, for it had longer roots and less Al in root tips, which indicates that Nipponbare is an Al tolerant variety of rice, while Kasalath an Al sensitive variety. It was also found in further studies that the expression of NRAT1, which is responsible for controlling Al uptake, was much higher in Kasalath than in Nipponbare, and the high expression is possibly the major cause leading to the high Al concentration in the root tips of Kasalath, an Al-sensitive variety of rice. Furthermore, it was also found that under Al stress, Nipponbare secreted more citric acids than Kasalath did and the expression of OsFRDL4, a gene controlling root secretion of citric acid, was significantly higher in level in the former than in the latter, which demonstrates that citric acid may possibly play a critical role in rice tolerating Al and the variety of rice controls secretion of citric acid by regulating expression level of OsFRDL4 gene. However, tests were also done on other four genes that may be related to Al tolerance, but no direct relationship was observed. So further studies need to be done.【Conclusion】 Nipponbare and Kasalath shows different Al tolerance in the present study. Nipponbare is an Al tolerant rice variety, while Kasalath an Al sensitive one. The difference in Al tolerance between the two varieties of rice may be attributed to the differences in expressions levels of NRAT1 and OsFRDL4.