Abstract: A rapid inhibition of root elongation is the main character of plants under aluminum (Al) stress. Two near isogenic lines (NIL) wheat (Triticum aestivum L.) ET8 (Al-resistant) and ES8 (Al-sensitive) were selected to study the mechanism of Al induced inhibition of root elongation. Effects of Al treatments on relative root elongation, root tip cell structure, contents of lignin and activities of enzymes such as phenylalanine ammonialyase (PAL), cinnamyl alcohol dehydrogenase (CAD) and peroxidase (POD) were investigated. The results show that the relative root elongation (RRE) of ET8 and ES8 are decreased after treated with 50 μmol/L Al for 6, 12 and 24 h. Longitudinal sections reveal that the cells in the elongation zone become flat, the cell wall is folded and ragged, intercellular space is decreased and occluded like gears after 50 μmol/L Al treated for 24 h. Damage to cell structure of ES8 is more serious than that of ET8. Both ET8 and ES8 showed an retardation of root cell elongation with the increase of treatment time (6, 12, 24 h) under 50 μmol/L Al, and there is a significant correlation between RRE and RCL (Relative Cell Length). The contents of lignin and the activities of PAL, CAD and POD are remarkably increased after the Al treatment (50 μmol/L 24 h). The above results indicate that Al-stress promotes the deposition of lignin by increasing the activities of PAL, CAD and POD, which accelerates the lignification of cell and leads to the decrease of cell wall extensibility and the inhibition of root growth. The content of lignin is higher in ES8 than that in ET8, and the damage to cell structure of ES8 is more serious than that of ET8, which induces the significant inhibition of root growth of ES8, and this might be the mechanism of ET8 could tolerate with Al-stress more than ES8.