Magnesiohulsite is a new mineral species found in China in 1985. It oc- curs in the magnesian skarn zone between biotitic granite (γ5(2))and Carbonife- rous dolomitic limestone in Dayi Mountain, Hunan Province. Quite a lot of its crystals have microtwins. The electron diffraction spots split regularly. Using the selected area electron diffraction method (SAED)，We have obta- ined the splitting electron diffraction spots patterns of [010], [141] acnd [121] zone axes. Through matrix analysis of the microtwins, it is determined that the two sets of splitting spots are of twin structure relationship, belong- ing to face-law twin with (101) twinning pla.ne.Its microshapes are demon- strated by bright field image. Different kinds of electron diffraction pat- terns show that the mineral belongs to monoclinic system with a0 10.7A，bo 3.01A，co 5.39A and β 94.5º.Charactezistic} of this twin structure and its crystallographic data are quite comparable with the result of X-ray single- crystal analysis. This paper provides a quite good mineralogical method for studying twins, especially microtwins of the mineral.