Abstract: In order to investigate the effect of Uniconazole application on tobacco seedling growth,the effects of Uniconazole application,at 7 concentration levels with 2 methods,on the root system,shoot growth and physiological indexes of tobacco seedlings in floating system were studied. The results showed that the matrix mixed with Uniconazole significantly inhibit the shoot growth of seedling;comparing with the control,the root-shoot ratio and root vigor of the treated seedlings averagely increased by 128.2% and 130.7%,respectively. The height of seedling was obviously negatively correlated to foliar Uniconazole spraying concentration. While for the stem girth,fresh weight,dry weight,chlorophyll content,vigor of root system and activity of nitrate reductase,Uniconazole application showed an effect of facilitation at low concentration and an effect of inhibition at high concentration. By spraying Uniconazole at the preparable concentration of 20mg/L,the root to shoot ratio reached 0.105 and the chlorophyll content,vigor of root system and nitrate reductase activity increased by 12.28%,38.39% and 154.33%,respectively. The development of tobacco seedlings is promoted by spraying 20 mg/L Uniconazole at the fifth true leaf stage.