The effect of light on production of cordycepin (3'-deoxyadenosine) was investigated using Cordyceps militaris. Among Cordyceps spp. tested, C. militaris KCTC 6064 was the best for cordycepin production, After 120-hr cultivation of C. militaris KCTC 6064 at in YMG broth with light (1,000 lux), 51.6 mg/l of cordycepin was produced. In general, production of cordycepin with light illumination was better than that without light. Cordycepin production under light illumination was enhanced up 96-hr cultivation, whereas after 96-hr, cordycepin production was decreased.