The effects of inlet air temperature, liquid feed rate and atomising air pressure on the yield, moisture content, crystal protein content and lethality of Bacillus thuringiensis powder during the spray drying of B. thuringiensis fermentation liquor were studied by an orthogonal experiment. The results of variance analysis indicate that the extent of influence of factors on the yield of B. thuringiensis is in the order: atomising air pressure greater than inlet air temperature greater than liquid feed rate; and on the moisture content, crystal protein content and lethality of B. thuringiensis is in the order: inlet air temperature greater than liquid feed rate greater than atomising air pressure. The mathematical models describing the relationship between spray drying parameters and the yield, moisture content, crystal protein content and lethality of B. thuringiensis were established. The optimum spray drying conditions determined by these mathematical models are: inlet temperature 1808C, liquid feed rate 60ml min 1, atomising air pressure 0 1MPa.The results of the validation experiment show that these mathematical models established by the orthogonal experiment are reliable.