Abstract: To further inspect physiological mechanisms of salt tolerant ability between different rice genotypes, and reveal salt tolerant ability mechanisms of rice which was regulated by ABA content in root during early salt stress stage, a solution culture experiment was conducted at phytotron in International Rice Research Institute (IRRI) from Oct. 2009 to Feb.2010, and strictly controlled the water supply, temperature, illumination and nutrients supplying. The results showd that biomass of the tolerant genotype (IR651) is higher than that of the sensitive genotype (IR29), and the Na+ content of the tolerant genotype is diluted by higher biomass, and is lower than that of the sensitive genotype. There is no significant difference of total Na+ absorption between the sensitive and tolerant genotypes. The root biomass of the tolerant genotype is higher than that of the sensitive genotype, and amount of Na+ in roots of the tolerant genotype is higher than that of the sensitive genotype, while the amount of Na+ in shoot of the tolerant genotype is lower than that of the sensitive genotype. Because of large amount of ABA in roots of the tolerant genotype during the early salt stress stage, the stomatal conductance and transpiration effective of the tolerant genotype are decreased significantly, the Na+ absorption is inhibited by decreasing of transpiration effective, and salt stress of rice plant is alleviated at a large scale. It can be concluded that compared with the sensitive genotype, the tolerant genotype has special root physiological characteristics, higher biomass and special ABA regulation mechanisms, thats why the tolerant ability of the tolerant genotype is higher than that of the sensitive genotype significantly under the saline condition