The experiments were carried out in growth chambers to investigate the effects of low temperature stress during flowering period on protective enzyme activities and membrane permeability of flag leaves using two rice cultivars including low-temperature tolerant cultivar 996 and low-temperature sensitive cultivar 4628. Optimal temperature(06:008:00，24 ℃；08:0010:00，26 ℃；10:0016:00，30 ℃；16:0019:00，26 ℃；19:0023:00，24 ℃；23:0006:00，22℃)and low temperature(06:008:00，19 ℃；08:0010:00，21 ℃；10:0016:00，23 ℃；16:0019:00，21 ℃；19:0023:00，19 ℃；23:0006:00，17 ℃)were respectively treated for seven days as two treatments after heading. The results showed that the activity of superoxide dismutase (SOD), catalase (CAT), peroxidase (POX) and ascorbic acid-peroxidase (AsAPOX) in flag leaves of two rice cultivars increased at early stage of low temperature stress, then decreased. The contents of thiobarbituric acid reaction substance(TBARS) and hydrogen peroxide(H2O2), superoxide anion(O2•) and the membrane permeability were increased remarkably as the period of exposure to low temperature lengthened. The protective enzyme activities in the cultivar 996 were higher while the contents of O2•, H2O2 and TBARS and the membrane permeability were lower than those in the cultivar 4628 under low temperature stress. Therefore, higher protective enzyme activities, which may resulted in lower contents of H2O2, O2• and TBARS and lower membrane permeability in rice flag leaf were the physiological basis of low-temperature tolerant rice cultivar.