Abstract: The effect of silicon (Si) on alleviation of aluminum (Al) phytotoxicity and corresponded mechanisms were investigated in this study by analyzing Al chemical forms in solutions and the distribution of Al in rice roots. The results indicated that the root growth was inhibited quickly and remarkably by Al treatment at pH 4.5,while this inhibition was alleviated by addition of Si in different forms and the elongation of root was approached or come up to those of control plants without Si addition. Al forms in the solution changed because of Si addition. A new Al-complex was found by investigating Al-forms in the solution with molecular-sieve chromatography and cation exchange resin, while the monomer Al concentration decreased. The ratio of exchangeable Al reduced from 78% to 48%, while the ratio of exchangeable Si increased from 0% to 20% with Si addition. Al in the roots was mainly accumulated in apoplastic tissues, especially in cell walls. In other hands, total Al content and Al content in the apoplast of roots increased, while Al content in the cell wall fraction decreased with the low molecular Si addition in the Al solutions. However, in the case of the high molecular Si addition to the Al solution, the contents of total Al, Al in the apoplast, in symplast fractions, and in cell wall decreased markedly.