Single crystal of hydrated sodium borate Na2B5O8(OH)center dot 2H(2)O has been grown with sizes up to 5 x 5 x 3 mm(3) under mild hydrothermal conditions at 180 degrees C. The structure is determined by single-crystal X-ray diffraction and further characterized by IR and TG analyses. It crystallizes in the orthorhombic space group Pna2(1), with a = 11.967(2) angstrom, b = 6.5320(13) angstrom, c = 11.126(2) angstrom, Z = 4, R1 = 0.0183, and wR2 = 0.0483. The crystal structure of Na2B5O8(OH)center dot 2H(2)O is made up of Na-O polyhedra, and B5O8(OH)(2-) polyborate anions. Transmittance spectrum is performed on the Na2B5O8(OH)center dot 2H(2)O crystal, which shows an absorption edge less than 190 nm in the UV region. The powder second-harmonic generation intensity measured by the Kurtz-Perry method indicates that Na2B5O8 center dot(OH)center dot 2H(2)O is about half that of KH2PO4 (KDP).