To improve mass transfer and enhance the yield for C(1,2) biodehydrogenation of steroid 11beta-hydroxyl medroxyprogesterone, we carried out the dehydrogenation reaction of 11beta-hydroxyl medroxyprogesterone in an oil-in-water (O/W) microemulsion by Arthrobacter simplex UR016. We studied the effects of system composition, dehydrogenation temperature and substrate concentration on microbial transformation. We formulated a suitable O/W microemulsion system with Arthrobacter simplex UR016 culture broth as aqueous phase, 10 g/L of edible oil as oil phase, 4 g/L of Tween-O80 and 7% (V/V) alcohol as surfactant and cosurfactant. The optimal dehydrogenation temperature was 33 degrees C. The results showed that in Tween-80/alcohol/edible oil/water microemulsion system, the hydrophobic steroid was solubilised and diffused effectively, with the maximum conversion rate of 88.6% at 46 h under 4 g/L substrate concentration, an increase of 66.2% compared to that in aqueous system. The C(1,2) biodehydrogenation of 11beta-hydroxyl medroxyprogesterone is more efficient in water-edible oil microemulsion system than in aqueous system.