Abstract: Tobacco seedlings were cultured in culture boxes floating on the low concentration nutrient solution to investigate arbuscular mycorrhizal fungal effects on plant growth，nutrition and selected enzyme activities. Three arbuscular mycorrhizal fungi (AM), two strains of Glomus intraradices Smith＆Schenck (BEG Number 141 and BEG Number 193) and Glomuse etunicatum Becker＆Gerdemann (BEG Number 168) were inoculated at the stages of sowing, fourth leaf and root occurrence. The results showed that the fungal effects varied with AM species and strains. The earlier the inoculation, the higher the infection rates, which ranged from 39.2% to 59.6% at sowing stage. The biomass, contents of phosphorus and chlorophyll, absorption of nitrogen, phosphorus and potassium by seedlings and activities of SOD in leaves, Chitinase and nitrogen reductase in roots were significantly increased by the inoculation of Glomus intraradices 141. Therefore, the inoculation of the suitable AM species or strains at sowing stage could be an efficient way to improve the qualities of tobacco seedlings in intensive culture of tobacco seedlings.