A novel ternary borate, sodium strontium pentaborate, NaSrB5O9, has been prepared by solid-state reaction below 800 ℃. The single-crystal X-ray structural analysis showed that NaSrB5O9 crystallizes in monoclinic, space group P21/c with a = 6.499（2）, b = 13.979（3）, c = 8.045（2）A, β = 106.92（2）°, V = 699.2（3）A^3, Z = 4, Mr = 308.66, Dc = 2.932 g/cm^3, μ = 7.804 mm^-1, F（000） = 584, R = 0.0264 and wR = 0.0621 for 2426 observed reflections and 146 variables. NaSrB5O9 is a layered compound containing double ring B5O11 building units composed of two BO4 tetrahedra and three BO3 triangles. Each B5O11 unit is connected to four other equivalent units through exocyclic oxygen atoms to form a two-dimensional ∞^2 [B5O9]^3- layer. Symmetry-center related layers are stacked along the b axis and held together by Na^＋ and Sr^2＋ cations via electrostatic interactions.
