Abstract: In order to increase the production of γ-polyglutamic acid (γ-PGA) from Bacillus siamese LW-1, three significant factors affecting yield of γ-PGA from the initial fermentation medium components were screened through the Plackett-Burman(PB) and the steepest climbing experiment based on the single-factor experiment to determine the optimal area of the response surface; the optimal medium components of Bacillus siamese LW-1 were obtained by using a three-factor and three-level Box-Behnken experiment. The optimal medium components for Bacillus siamese LW-1 producing γ-PGA were: sodium glutamate 86.71 g/L, sodium citrate 17.94 g/L, MgSO₄·7H₂O 2.11 g/L, glycerol 25 g/L, KH₂PO₄ 1.4 g/L, (NH₄)₂SO₄ 14 g/L, MnSO₄ 0.075 g/L, CaCl₂ 0.1 g/L, FeCl₃·6H₂O 0.04 g/L. Under this optimum condition, the yield of γ-PGA was 44.78 g/L which was 1.93 times that of before optimization(23.26 g/L ), and it was very close to the maximum value 45.91 g/L predicted by the theory.