A Heusler-type Ni2.18Mn0.82Ga single crystal shows a shape memory effect. It makes a thermo-elastic martensitic transformation at around 340 K, which is coincident with a Curie temperature. We made a synchrotron radiation white X-ray diffraction of the single crystal by changing the temperature from 400 K to 103 K. We observed the change of Laue spots following the transformation. As a result of experiment, the single crystal shows one cubic Heusler structure at 400 K. The direction of the crystal growth is along cubic  direction. In the process of decreasing temperature, many tetragonal structures with small volume of different axial direction become to appear. At room temperature the transformation almost finishes and two tetragonal Heusler structures which are twin each other remain. One of them is nearly the same as the structure at the beginning room temperature. The direction of the crystal growth is  of this tetragonal structure. We found that the twinning plane of the tetragonal structure is (011) plane.