Abstract: From all tobacco endophytic bacteria, 21 strains were tested for their ability to reduce NO3- to NO2-, the endophytic characteristic of strain K18 which didn’t have the ability on nitrate reduction was confirmed and was tested for the ability on TSNA reduction in greenhouse and in field. Tobacco were treated with K18 on different growing stage, such as root dipping during transplanting, spraying on tobacco leaves during branching and luxuriant growing stage, spraying a week before harvest, and chemical contents of TSNA in the leaves were examined. Results showed that the amount of TSNA in the leaves was different responding to different treatment of bacteria K18 on different tobacco variety. The total amount of TSNA was reduced at all treatments with K18 in greenhouse and in field, but the reduction efficacy was related to the tobacco variety. TSNA reduction was suspected to relate to the total bacterial number in tobacco leaves, The total bacterial number in tobacco leaves treated with K18 during the branching stage and luxuriant growing stage was higher than that control, and the TSNA reduction rate reached 23.7% and 9.1% respectively. NNN, a major content of TSNA was reduced effectively at the treatments by K18 either in YunYan87 or in TN86. And it was higher in TN86 than that in YunYan87 In terms of YunYan87 planted in field, the reduction rate could reach 50.1% and 39.6% respectively at the treatment of spraying luxuriant growing stage and root dipping stage compared to the treatment without K18.And as to TN86 grew in field, the reduction rate reached 52.6%, 51.1% and 44.7% respectively at the treatment of spraying luxuriant growing stage, the treatment of spraying at curing and the treatment of root dipping compared to control.