Diosgenin is an important raw material in steroid hormone and widely used in pharmaceutical industry. The traditional method for diosgenin production is acidolysis, which causes serious pollution. In order to obtain a cleaner and more efficient approach of diosgenin production, a strain of Gibberella intermedia WX12 (the sexual stage of Fusarium proliferatum) was screened from the strains deposited in our laboratory. This strain converted saponins in Dioscorea zingiberensis C.H. Wright (DZW) into diosgenin. The conversion medium was optimized by statistical experimental design. The optimized conversion medium was as follows (g/L): glucose 20.6, yeast extract 5.0, NaCl 1, K2PO4 3, ZnSO4 x 7H2O 1.5 and saponins 3. Under the optimal conditions, the yield of diosgenin achieved to (31 +/- 0.3) mg/g DZW, which was 3 times higher than that of the original medium.