Abstract: The roots of rice seedlings were treated with different concentrations of Methyl Viologen(MV)solutions,and the effects of different oxidative stress on the chlorophyll fluorescence parameters of different cold-sensitive cultivars at normal and low temperature were studied.The results showed that at normal temperature,Fv/Fm,Φ_PSⅡ and qP of rice seedlings were higher,whereas the parameters were lower treated by higher concentration of MV in longer time than that in their controls,and the cultivar with cold-tolerance had higher NPQ.At low temperature,Fv/Fm increased only when treated by MV₅ in shorter time,Φ_PSⅡ and qP both decreased with all treatments,and NPQ increased with treatments of less concentration MV in shorter time.This suggests that less oxidative stress may promote the photosynthetic capacity of rice,but oxidative stress could prick up the hurt to rice at low temperature,and the cold-tolerant cultivar showed stronger cold-tolerance because it could relieve the excessive oxidative hurt through maintaining higher linear electron transport rate and heat dissipation ability.