The crystal structure of high clinoferrosilite (FeSiO3) with C2/c symmetry after the transition from orthoferrosilite at 1025°C was determined at 1050°C using a high-temperature four-circle diffractometer. The cell constants are a = 9.928(1), b = 9.179(1), c = 5.338(1)Å, β = 110.20°(1), V = 456.5(1)Å3. The Fe2 polyhedron can be considered as six coordinated because the next two oxygens are too distant (3.191 Å) for effective coordination with the Fe2 ion. The mean Si–0 bond length is 1.634Å and the mean Fe–O bond lengths in the M1(6) and M2(6) polyhedra are 2.176 and 2.300Å, respectively. The polyhedral volume calculation reveals that the six-coordinated M1 polyhedron (13.40Å3) is larger than the six-coordinated M2 polyhedron (11.93Å3). This is related to the large distortion of the Fe2-polyhedron in the high clinoferrosilite structure. A similar tendency has been observed for the high clinopyroxene structures reported previously.