Single crystals of the β-Ca₃B₂SiO₈ new monoclinic modification have been obtained by cooling the melt of a stoichiometric composition. The crystal structure has been determined from the single crystal X-ray diffraction data and refined with R = 0.059 (wR = 0.069) in the monoclinic space group P2₁/m. The thermal behavior of the synthetic borosilicate has been studied. At 472 ± 5°С, a reversible phase transition of the first order occurs, leading to the formation of the orthorhombic α-Ca₃B₂SiO₈ modification. The thermal expansion of α- and β-modifications of Ca₃B₂SiO₈ is anisotropic: (α₁₁ = 15, α₂₂ = 16, α₃₃ =–1, α_V = 30 × 10^–6°С–1) and α₁₁ = 9, α₂₂ = 28, α₃₃ = 1, α_V = 38 × 10^–6°C–1, respectively.