Mirabilite (Na₂SO₄⋅10H₂O) is one of the representative phase change materials. Temperature dependent structural behavior of mirabilite was investigated by using single-crystal X-ray diffraction (XRD) technique in the temperature range between 213 and 303 ​K. Thermal expansion of mirabilite was mainly ascribed to the expansion of two Na(H₂O)₆ octahedra constituting the ribbon structure along the c-axis. Three O atoms out of four apices of SO₄ tetrahedron were split into two partially occupied O sites. The resulting disordered arrangement gave two orientations (major and minor) for the SO₄ tetrahedron. The SO₄ tetrahedron of the major orientation had the smaller volume than that of the minor orientation. The volume of the major SO₄ tetrahedron showed a decreasing trend as increasing temperature. This shrinkage of the major SO₄ tetrahedron would induce oxygen diffusion from the major SO₄ tetrahedron to the minor SO₄ tetrahedron. As a result, the O site occupancies belonging to the minor SO₄ tetrahedron increased from 0.17 to 0.27. Crystal structure of mirabilite consists of infinite ribbons of edge-sharing Na(H₂O)₆ octahedra alternating with chains composed of SO₄ tetrahedra and two H₂O molecules to form sheets parallel to the b-c plane. Since the fundamental structure of mirabilite is essentially the same as that of thenardite (Na₂SO₄), phase change from mirabilite to thenardite immediately occurs after dehydration without formation of any intermediate phase.