Fe-Ga alloys are used for practical magnetostrictive materials. The understanding of mechanical properties under magnetic field is very important for their application to vibration power generation devices. Especially, Poisson’s ratio is one of key parameters because it is strongly related with the volume change of materials. In the study, we investigated actual behavior of Poisson’s ratio due to magnetostriction, instead of just the mechanical one. The sample was cube-oriented Fe-18mol%Ga single crystal disc. Static magnetic fields were applied in various directions parallel to (001). Strain values in various directions were measured by strain gauge and X-ray diffraction. And then, tri-axial strain analysis for single crystal was carried out. Theoretically, Poisson’s ratio due to magnetostriction is known to be 0.5, (volume conservative), value is close to the mechanical one, 0.45. On the other hand, we found that Poisson’s ratio exhibits anisotropic behavior despite the elastic constants are equivalent in  and . This phenomenon is attributable to the magnetic domain structure under zero magnetic field. In this presentation, we will discuss this apparent Poisson’s ratio considering the volume strain and magnetostriction comprehensively.