A new magnesium borate MgO·B2O3·4H2O was synthesized by the method of phase transformation of double salt and characterized by XRD, IR spectra and TG. The structural formula of this compound was Mg·[B2O(OH)6]·H2O. The enthalpy of solution of MgO·B2O3·4H2O in HCl (1.0492 mol·L−1) was determined. With the incorporation of the standard molar enthalpies of formation of MgO (s), H3BO3(s), and H2O (1), the standard molar enthalpy of formation of ‐ (3135.31 ± 1.68) kJ·mol−1 of MgO·B2O3·4B2O was obtained. Thermodynamic properties of this compound were also calculated by group contribution method.