Whisker shaped magnesium oxysulfate (MOS, 5Mg(OH)2 MgSO4·3H2O) was synthesized form MgSO4·7H2O and triethanolamine (TEA) using a hydrothermal method at 80 ℃ for 24 h. An aqueous solution of 2 M MgSO4·7H2O was used as the precursor and TEA was used as the chelating ligand. The experimental results indicate that the TEA promotes the formation of the desirable whisker morphology during the hydrothermal synthesis. The Mg atom coordinates with an O atom of SO42- in another layer to form a multi layer crystal structure. The thermal analytical curve of this compound was recorded, and its decomposition mechanism was discussed. In particular, the use of this TEA solution led to the simple and economical synthesis of magnesium oxysulfate whiskers with lengths of 100-200 μm and diameters of 80-200 nm. The product characteristics were investigated by making XRD, FT-IR, TG, FE-SEM, EDS and TEM observations.