Abstract: The effects of sodium nitropprusside(SNP,an exogenous nitric oxide donor) on photosynthesis-light response curve and CO2 response curve in seedlings of tomato(Lycopersicon esculentum Mill.)were investigated under 100 mmol/L NaCl stress condition.The results showed that net photosynthetic rate(Pn),transpiration rate(Tr) and stomatal conductance(Gs) were significantly increased,intercellular CO2 concentration(Ci) were markedly decreased,light saturation point(LSP),photosynthetic rate at light saturation point,apparent quantum yield(AQY),CO2 saturation point(CSP),carboxylation efficiency(CE) and maximum RuBP regeneration rate were markedly increased,light compensation point(LCP),CO2 compensation point(CCP) were significantly decreased by the treatment of exogenous nitric oxide under NaCl stress.In addition,the chlorophyll contents were significantly enhanced.The above results indicated that exogenous nitric oxide donor(SNP) could improve photosynthesis of tomato seedlings under NaCl stress and increase salt tolerance of plants.