Abstract Reactive oxygen species (ROS)-related membrane lipid peroxidation in the root of Arabidopsis thaliana was fluorescently visualized and investigated under salt stress. In the control roots without salt stress, more fluorescence was observed in the elongating region than in the meristematic region. Salt stress of 100 mM NaCl enhanced the fluorescence in both, indicating that salt stress-induced ROS, and consequently membrane lipid peroxidation. In transgenic tobacco glutathione S-transferase over-expressing Arabidopsis (the parB plants), less fluorescence was observed than in the non-transgenic control plants. In the salt-stressed parB plant roots, the fluorescent brightness was reduced to 46% of that of the non-transgenic plant in the meristematic region. However, the inhibition of root growth was not improved in parB plants under salt stress at pH 5.7. That is, 100 mM of salt stress reduced the root growth to 40% or less both in the parent control plants and the parB plants. The root tissue osmotic pressure was almost the same between the two tested lines, which may be one of the reasons why no difference was observed in root growth between the two lines. These results suggested that salt stress-induced oxidative stress, and the introduction/over-expression of a glutathione S-transferase gene may have reduced the amount of ROS but the removal of ROS was not sufficient to effect salt tolerance because salt stress also caused an osmotic imbalance reducing the root growth.