Abstract: This paper discussed some physical and chemical properties of trihydrated MgCO3 and its hydrolytic product-basic MgCO3 is obtained from NH4HCO3 reacting with MgCl2 solution. The trihydrated MgCO3 and basic MgCO3 are examined with differentia thermal gravimetric, differential thermal, infrared spectra and X-Ray diffraction analyses. Besides, we find out that the two molecules of crystal water in MgCO3·3H2O have different bonding abilities, that the structural formulae of MgCO3·3H2O and 3MgCO3·Mg (OH)2·4H2O are Mg(HCO3)(OH)·2H2O and Mg(HCO3) (OH)·Mg(OH)2·H2O respectively; and that the exothermic peak of basic MgCO3 at 515℃ has certain relationship with physical property of the product itself.