By measuring chlorophyll fluorescence, the effects of NaCl treatment on the maximal efficiency and heat tolerance of PSII were examined in leaves of Rumex seedlings. NaCl 200 mmol/L treatment had no effect on the maximal efficiency of PSII, but increased the heat tolerance of PSII in Rumex leaves. Compared with control leaves, the heat stress-induced decrease in F(v)/F(m) and the increase in F(k)/F(j) ratio were less in NaCl-treated leaves. In addition, the heat stress-induced decrease in photochemical quenching (q(P)), the efficiency of excitation energy capture by open PSII reaction center (F(v)' /F(m)') and the quantum yield of electron transport (PhiPSII) were less in NaCl-treated leaves. Moreover, the increase in the Q(B)-non-reducing PSII reaction center content was less in NaCl-treated leaves than in control leaves. Discussion was made on the possible mechanisms of the increase in the heat tolerance of PSII in NaCl-treated leaves.