Mouse embryonic stem cells（ES cells）are pluripotent and able to differentiate into skeletal muscle cells. ln the present study we analyzed the formation process of skeletal muscle cells from mouse ES cells, expression of transcription factors critical for skeletal muscle differentiation and or α-actinin, and sarcomere formation by immunohistochemistry for myosin heavy chain. Results were then compared with those of skeletal muscle differentiation using C2C12 mouse myoblast cell line．Although undifferentiated ES cells expressed neither transcription factors nor a α-actinin mRNAs, myoblasts and differentiated skeletal muscle cells expressed these mRNAs as well as differentiated cells from C2C12. ES cells at passage 12 to 14 showed the highest potential for muscle differentiation. Both muscle cells from C2C12 and ES cells expressed a α-actinin, however, only ES cells formed sarcomere consisting of regular band patterns and showed muscle contractions. Myotubes from C2C12 developed few sarcomeres and did not contract. We discussed the factors required for sarcomere formation, and finally indicated that skeletal myogenesis from mouse ES cells is a good in vitro developmental model of skeletal muscle involving early stages before commitment of skeletal muscle cell lineage and sarcomere formation in late stages.