Abstract: Development of Pleurotus geesteranus Singer mycelia in the submerged medium containing 3 μg added selenium (Se) per mL was studied in a 3×10 L auto-control fermentor. Biological characteristics, culture environment and medium nutrition including the diameter and density of the mycelial spheres, as well as the pH, DO, total soluble sugar and protein of the medium were monitored to determine the effect of the selenium addition. The results showed that (a) the mycelial growth was basically the same with or without the Se addition, (b) the biomass as well as the sphere’s diameter and density increased with incubation time, and (c) a significant correlation existed among the factors. At the end of 72h culture, the biomass in CK and Se groups were 1.92 and 1.81 mg·mL-1, respectively; the Se content of mycelium in the treatment group was 45.37 mg·kg-1 with an Se bioconversion efficiency of 53.37%. During incubation, the total soluble sugar in the culture medium slowly increased initially followed by a decline, but the total soluble protein remained constant on a decreasing trend.