Oncostatin (OSM) is an important cytokine which has diverse biological activities. It may be useful in basic scientific and clinical studies. OSM was efficiently expressed by cloning the OSM gene into GST fusion expression vector. The level of expression of the fusion protein was about 50% of the total cell protein by SDS-PAGE analysis. The percentage of fusion protein in soluble protein could be 15% when the induction was in low temperature. The purity of purified GST-OSM with an affinity chromatography column was about 90% after the inclusion bodies had denatured and renatured. The result of activity study showed that the first two amino acids in N-terminal of the OSM were important to the biological activity of OSM.