he concentrated MgCl2 -eutectic brine containing borate was stepw1se cooled in stir-ring to 0℃` - 10℃and - 20℃ respectively' The Salts crystallized out at each temperatare were identified by chemical analysis,thermal analysis and x-ray powder diffeaction. The re-sults showed that the highly hydrated Mg-chloride was cyrstallized out at - 20℃ and MgO . 3B2O3. 7H2O will be crystallinzed out when the brine is let from room temperture to -20. 5℃ for different times.