Abstract: Liquid fermentation media was optimized for promoting the yield of cordycepin by cordyceps militaris mycelium using response surface methodology and the corresponding yield of mycelium were measured.Firstly,factors playing important roles in the cordycepin production were selected based on Plackett-Burman design. Then the path of steepest ascent was undertaken to approach the optimal region of the cordycepin production. Finally,the optimal levels of those main factors were further optimized by using Box-Behnken design. All above experiments were repeated to analize the correlation between cordycepin and mycelium production. Fermentation conditions were temperature 25 ℃,dark,rotation speed 160 r/min,natural p H,inoculating amount 5 m L/100 m L. Optimal media was KNO₃0.04 g/100 m L,yeast extract 1.50 g/100 m L,Fe SO₄·7H₂O 0.03g/100 m L,KH₂PO₄0.2 g/100 m L,Glucose 3.82 g/100 m L,Zn SO₄·7H₂O 0.06 g/100 m L,Mg SO₄·7H₂O 0.13 g/100 m L,Vitamin B10.08 g/100 m L. Cordycepin production reached 852.621 μg/m L. Then,using the optimal media on the same conditions,cordycepin production was optimized with 8 days' shaking-flask and 10 days' static culture and reached 936.225 μg/m L. The results of relationship between the yield of cordycepin and mycelium showed that when the yield of mycelium was less than 0.857 g/100 m L,it was more efficient that mycelium produced cordycepin. When the yield of mycelium was more than 1.708 g/100 m L,the yield of cordycepin started descending. Production of cordycepin and mycelium at 8th day had significant correlations.