The liquid-solid phase diagram of thermodynamic non-equilibrium state of system at 0 °C was given. This result was based on investigation of the crystallization processes of MgO-nB_2O_3- 18%MgSO_4-H_2O system at 0 ℃, identification of the compositions of solid phases by physical method and chemical analysis, and determination of the thermodynamic equilibrium point. There are four fields in this diagram, corresponding to H_3BO_3, MgO· 2B_2O_3·9H_2O (Hungchaoite), MgO·3B_2O_3·7.5H_2O (Meallisterite) and 2MgO·3B_2O_3·15H_2O (Inderite), respectively. Th research results show that MgSO_4 and MgCl_2 have different influence on the crystallization of Mg-borates.

Key words: Mg-borate, PHASE DIAGRAM, CRYSTALLIZE, KINETIC STUDY, MAGNESIUM CHLORIDE, MAGNESIUM SULFATE