Abstract: The effects of silicon on ultrastructure and active oxygen metabolism in root mitochondria were studied with a tomato（Solanum lycopersicum）cultivar‘Jinpeng 1’in cultured hydroponics under PEG-6000 simulated drought. The results showed that with the extension of drought stress，the root activity of tomato plants gradually decreased. Malondialdehyde（MDA）content，mitochondrial superoxide anion（）productivity rate and hydrogen peroxide（H2O2）content gradually increased，but the activities of superoxide dismutase（SOD），peroxidase（POD）and catalase（CAT）rapidly increased in the early stage of drought stress，and then rapidly decreased after peak. The application of silicon significantly effected the change of the related indicators caused by drought stress. After 12 days of drought stress，the content ofMDA，and H2O2 in roots of tomato with silicon were lower than that of tomato without silicon by 25.14%，8.34%，20.25%，respectively. But root activity，SOD activity，POD activity and CAT activity were higher by 139.62%，243.19%，65.53%，125.55%，respectively. In addition，the application of silicon not only reduced plasmolysis of tomato root cells，but also relieved the damage of mitochondria，nuclei and biofilms.

Key words: tomato, silicon, drought, cell ultrastructure, mitochondria, active oxygen metabolism