The effects of sodium nitroprusside(SNP, an exogenous NO donor)on aluminum(Al)toxicity in tobacco were studied by investigating plant growth, reactive oxygen content, photosynthesis, chlorophyll fluorescence characteristics, photorespiration and antioxidative enzymes in leaves of tobacco cultivar Yunyan 100(Al-tolerant)and Yunyan 105(Al-sensitive)with solution culture method. The results showed that under Al stress, the leaf chlorophyll content, net photosynthetic rate(Pn), maximal photochemical efficiency(Fv/Fm), quantum yield of electron transport at PSII(?PSII)and coefficient of photochemical quenching(qP) were dramatically decreased, which increased excess energy of PSII photochemistry leading to higher accumulation of O2? and H2O2, and lower plant biomass in both genotypes. Under Al stress, application of exogenous NO dramatically increased the leaf chlorophyll content, Pn, Fv/Fm, ?PSII, qP and plant biomass, decreased excess energy of PSII photochemistry and accumulation of O2? and H2O2 , the effects were more predominantly in Yunyan 105 than in Yunyan 100. Furthermore, the coefficient of non-photochemical quenching, photorespiration rate, and activities of superoxide dismutase and ascorbate peroxidase were increased in both genotypes in the presence of NO under Al stress. These results indicated that application of exogenous NO could enhance the tolerance of tobacco to Al stress mainly by increasing of the photochemical reaction capacity, heat dissipation, photorespiration and activities of antioxidative enzymes, which prevents or eliminates ROS accumulation, and the effect is greater in the Al-sensitive genotype than in the Al-tolerant genotype.