Reaction of magnesium sulfate with aqueous ammonia in a hydrothermal condition (150−180 °C) produced a fibrous magnesium hydroxide sulfate hydrate (MHSH) with a composition corresponding to a stoichiometry of 4.34Mg(OH)₂·MgSO₄·xH₂O. Chemical composition analysis showed that the quantity of x was more likely to be 2 rather than the well-known 3. The MHSH whiskers were about 0.3−2 μm in diameter and 40−100 μm in length, with an aspect ratio no less than 100. Selected area electron diffraction (SAED) and X-ray diffraction patterns (XRD) showed the whiskers were well-crystallized with an orthorhombic structure. An interesting superlattice was observed from the ED patterns but this superstructure was unstable under the radiation of a high-energy electron beam especially under HREM conditions. Several techniques such as XRD, TEM, EDS, HREM, XPS, TGA-DTA, FTIR, etc. were used to characterize the structural features and thermal behavior of the experimental products. Our sample of fibrous MHSH was found to lose two hydration waters at about 330 °C, to further decompose at higher treatment temperature, and to transform finally into MgO near 1000 °C.