Abstract: The studies on fermentation of Bacillus thuringiensis showed that the fermentation beer produced by 7 strains of this bacterium grown in one fermentation medium and the fermentation beer produced by 1 strain of this bacterium grown in seven fermentation media represented different toxicities against the test insect.The toxicity against the test insect was related to the sensitivity of insect and to different strains of Bacillus thuringiensis,and also to the composition of fermentation media.However the toxicity was not correlated with spore count. Varies of N.C and C/N of media during fermentation process did not relate to the toxicity of fermentation production against the test insect. Therefore,the potency of the fermentation production of Bacillus thuringiensis can be determined only by bioassay.