Abstract: A simple oil water emulsion system was developed for highly efficient biotransformation of phytosterols to 9α hydroxyandrost 4 ene 3, 17 dione (9 OH AD). Results indicated that mechanical shearing force generating shear force and addition of Tween 80 as dispersant were the essential factors to form a stable emulsion system. And the ratio of cereal oil was related to the effective load of system for phytosterols concentration. In the emulsion system, a higher biomass was got at an evenly dispersed state. Application of the emulsion system accelerated biotransformation process at higher phyosterols concentration of 20 g/L phytosterols with 64.06% molar yield and 1.28 g/(L·d) productivity of 9 OH AD. After biotransformation, the major by products partitioned in the oil phase would be removed with oil, and the purity of 9 OH AD by the chromatography area normalizing increased from 66% to 90%.