There are many microorganisms that are used for control of plant pests in the world. Among them, Bacillus thuringiensis has a wide range of activity and is more effective. The most important crop hosts of which H. armigera is major pest is tobacco. The insect is polyphage and there is a reported amount of 70 host species in world. The larvae feed on leaves, bolls, fruits and seeds of plants. In biological control method, using disease factors, bacteria has been used commercially more than others. At present, three species of bacteria are used for pest control, of which one is Bacillus thuringiensis used to control Lepidoptera larvae. In this study, experimental design was 4x5 meters and 2 meters distance together (178-2 Basma tobacco) in 2006-2007. Treatments involved three preparations of Bt, and control (check). For this purpose, experimental design was split plot in randomized complete design with four replications. In this study, different larval instars with three levels (1, 3 and 5) as a main factor and with four levels (three preparations of Bt & control) as a sub-main factor were used. Then, in each experimental unit, 20 larvae were randomly selected and the effects of Bt on the mortality were evaluated. Mortality percent calculated by Henderson-Tilton formula. The analysis of variance showed that there was a significant differences between the different larval instars and the preparations of Bt. Results showed that rate of larval mortality occurred at the preparation of Bt (III) 54.14% preparation of Bt (II) 44.3%, preparation of Bt (I) 35.07% & control 14.6%. Experimental results showed that with increasing of instar larval, the susceptibility to all of the preparation bacteria (Bt) were decreased.