Abstract: The effects of different concentrations (5, 10 and 20 nmol/L) of exogenous auxin (IAA) on growth, photosynthetic characteristics and endogenous IAA of flue-cured tobacco seedlings were studied by hydroponic experiments. The results showed that, compared with the control, the exogenous IAA treatments significantly increased the biomass of root and aerial part of flue-cured tobacco seedlings by 89.43% and 73.91% respectively. The total length, surface area and volume of roots in seedlings treated with different IAA concentrations increased to varying degrees. Among them, the 10 nmol/L treatment significantly increased the total length, total surface area and volume of the roots, while the average roots diameter of the seedlings after different IAA treatments showed no significant difference. The net photosynthetic rate and stomatal conductance of flue-cured tobacco seedlings were significantly increased by different exogenous IAA treatments, but no significant difference of intercellular CO₂ concentration, transpiration rate and stomatal limitation value were observed. Exogenous IAA treatment significantly increased the content of endogenous IAA in the root and aerial part of flue-cured tobacco seedlings. It was concluded that the application of exogenous auxin promoted auxin synthesis of aerial part in flue-cured tobacco seedlings and polar transportation to the root system, thus promoting growth and development of tobacco.