Suanite, a mineral of magnesium borate, rarely occurs in nature. We have studied the conditions for its formation and stability by laboratory synthesis and hydrothermal experiments. The experiments were carried out in cold-seal vessels at the temperature of 350-500℃ and pressure of 200-1000 bars, with MgC03 as a starting material and H3BO3, Na2B40, and H3BO3 +NaCI as reactive solutions. Suanite, kotoite, szaibelyite and ludwigite were formed under these conditions. The experimets show,(1 )suanite occurs in a high-boron and magnesium, low-iron hydrothermal environment. It can be formed by reaction of MgCO3 with 1-3 M H3B03 or Na2B4O7 solutions under temperature of 500-600° C and pressure of 200一1000 bars. In the case of reaction of MgCO3 with relatively dilu- ted H3B03 solution and under the same P-T conditions, the product is kotoite instead of suanite.(2 ) Suanite is formed and stable at relatively high tempe- rature(500-600°)and may change into szaibelyite by hydration when tempera- ture drops below 450℃.(3 )Suanite is formed under a low CO2 partial pressure and a wide range of pH values and is stable when pH=5-9. From the results of the experiments, it is considered, that suanite may not be a typical skarn mineral. It seems most probable that suanite is formed by high-temperature metamorphism of sedimentary formations of hydrous magne- sium borates and boron-bearing magnesium carbona,tes. The hydrous borate mi- nerals, when undergo high-grade regional or contact metamorphism, would chan- ge into anhydrous borate minerals such as suanite and kotoife through dehy- dration. During the contact metamorphism of the magnesium-carbonate forma- tion, boron form formation iself and from endogenic hydrothermal solutions may also react with carbonates to form suanite. Therefore, it's reasonable that sua- nite is formed by hightemperature metamorphism of sediments.