In this work, a new way of measuring the volume fraction of e martensite in Fe-based alloys has been proposed. Since the specific volume of ${varepsilon}$ martensite, depending on alloy composition, is smaller than that of austenite i.e ${gamma}$ phase, volume expansion takes place during ${varepsilon}{ ightarrow}{gamma}$ reverse transformation. As the amount of the volume expansion is proportional to the product of specific volume difference times the volume fraction of ${varepsilon}$ martensite, the volume fraction of ${varepsilon}$ martensite can be calculated by measuring the volume expansion and the specific volume difference. Such a relationship was confirmed in Fe-21Mn and Fe-32Mn-6Si alloys which undergo ${gamma}{ ightarrow}{varepsilon}$ martensitic transformation on cooling and by cold rolling, respectively. It was also found that the former has isotropic ${varepsilon}$ martensite while the latter has anisotropic ${varepsilon}$ martensite.