The relationship of standard molar enthalpy of formation between nano and bulk BaMoO₄ was obtained by designing a thermo chemical cycle according to thermodynamic potential function method, and the heat of reaction that nano BaMoO₄ reacted with HCl could be gotten by microcalorimetry, based on the known standard molar enthalpy of formation, which was －1507.5 kJ•mol^－1 of the bulk BaMoO₄, standard molar enthalpy of formation for octahedron BaMoO₄ nanostructures prepared by inverse microemulsion at 298.15 K was calculated as (－336.62±0.33) kJ•mol^－1, the different value of two kinds of materials proved that the energy state of nano materials was higher and more unstable.