Al-tolerant wheat (Triticum aestivum L.) Atlas 66 and Al-sensitive wheat Scout 66 were studied to elucidate the differences on cell structure and ROS metabolism in root tip under Al stress.The results showed that both Atlas 66 and Scout 66 showed a retardation of root cell elongation after treated with 50.0 mol/L Al for 6,12 and 24 h.RRE (relative root elongation) and RCL (relative cell length) was well correlated (R2=0.919).Root tip cells of Atlas 66 and Scout 66 were damaged with different degree:Longitudinal sections revealed that the epidermal cells in the elongation zone became flat,intercellular space decreased after 24 h of 50.0 mol/L AlCl3 treatment.Damages to cell structure on Scout 66 were more serious than that on Atlas 66.The results indicated that the inhibition of cell elongation was due to the damage of cell structure,thus inhibited the elongation of root.After the treatment of 50.0 mol/L AlCl3 for 24 h,the activity of superoxide dismutases (SOD),catalase (CAT),ascorbate peroxidase (APX),peroxidases (POD) and glutathionereductase(GR) in Atlas 66 and Scout 66 increased by 8.2%,82.0%,44.0%,25.0%,23.5% and 23.3%,33.6%,26.9%,32.5%,14.8%,respectively.The results indicated that increased activities of enzymes in ROS metabolism were induced by the application of Al and that difference in ROS metabolism can cause the Al-tolerant difference among different Al-tolerant wheat varieties.