Abstract: The thermal expansion of magnesium borate (Mg₃B₂O₆) is studied by the method of X-ray thermodiffraction in the temperature range 25–1100°C. The coefficients of thermal expansion are calculated as follows: α_a = 13.0(5), α_b = 8.5(5), α_c = 8.0(5), α_V = 30(1) × 10^–6°С⁻¹ at 25°C; and α_a = 20.0(5), α_b = 13.0(5), α_c = 13.0(5), α_V = 46(1) × 10^–6°С⁻¹ at 1100°С. The direction of the maximal thermal expansion (α_a) is perpendicular to the preferred orientation of the BO₃ triangles.