Single crystals of potassium pentaborate dihydrate KB5H4O10. 2H2O (KBH) of dimension (10 × 5 × 2 mm3) were successfully grown through the solution growth technique under slow evaporation condition. The crystal structure of potassium pentaborate dihydrate was determined by single crystal X-ray diffraction and the material was crystallized in the orthorhombic system with space group Aba2 with lattice parameters a = 11.13 (Å), b = 11.23 (Å), c = 9.10 (Å), α = β = γ = 90°, volume = 1137 (Å)3. The presence of BO3, BO4 and water molecules were confirmed using Fourier transform infrared (FT-IR) vibrational spectroscopy. The ultraviolet (UV) - visible spectrum reveal the optical and electronic transition properties of the grown crystal. The optical absorption edge of the grown crystal is 210 nm with energy gap of 4.96 eV. The thermogravimetric (TG) and differential scanning calorimetry (DSC) studies showed that the material was stable up to 106 °C, which indicated that the grown crystal could be effectively used for various device applications below the temperature. Vickers microhardness is performed on the grown crystal in the load range from 10 to 50 g. The variation of hardness number with load represents the normal indentation size effect. Mechanical parameters such as yield strength, elastic stiffness constant, fracture toughness, brittleness index and crack propagation of single crystal are discussed in detail. The dielectric behaviour of the single crystal over an extensive range of frequencies and temperature is described. The second harmonic generation (SHG) signal from the KBH crystal is found to be 1.23 mV. © 2018