The molar dissolution enthalpy of the reactants and the products of the coordination reaction of erbium nitrate with alanine hava been measured by classical solution calorimetry,the calorimetric solvent is 2mol· L- 1 HCl solution.The standard molar reaction enthalpy of this coordination has been determined by a new chemical circle ,and =17.800kJ· mol- 1.From the results and other auxiliary quantities ,the standard molar enthalpy of formation of the new complex Er(Ala)4(NO3)3· H2O has been calculated to be [Er(Ala)4(NO3)3· H2O,s, 298.2K]= - 3818.2kJ· mol- 1.