Abstract： It has been known that estrogen-17β stimulates proliferation of mouse embryonic stem (mES) cells. To explore the function of another steroid hormone progesterone, we used MTI" method and BrdU incorporation assay to obtain growth curves, clone forming assay to detect the propagation and viability of individual mES cells, Western blot to test the expression of ES cell marker gene Oct-4, fluorescence activated cell sorter (FACS) to test cell cycle, and real-time PCR to detect the expressions of cyclins, cyclin-dependent kinases and proto-oncogenes. The results showed that progesterone promoted proliferation of mES cells. The number of clones was more in progesterone-treated group than that in the control group. The expression of pluripotency-associated transcriptional factor Oct-4 changed little after progesterone treatment as shown by Western blot, indicating that most of ruES cells were in undifferentiated state. The results of FACS proved that progesterone promoted DNA synthesis in mES cells. The proportion of mES cells in S+G_2/M phase was higher in progesterone-treated group than that in the control group. Cyclins and cyclin-dependent kinases, as well as protooncogenes (c-myc, c-fos) were up-regulated when cells were treated with progesterone. The results obtained indicate that progesterone promotes propagation and viability of ruES cells. The up-regulation of cell cycle-related factors might contribute to the function of progesterone.