Abstract: The hydrothermal formation of hemi-hydrate calcium sulfate whiskers (CaSO4·0.5H2O) with high aspect ratio was discussed in this paper,using various gypsums (CaSO4·2H2O) as the raw materials.The influence of the properties of the gypsums on the morphology of the hydrothermal products was studied.The experimental results showed that hemi-hydrate calcium sulfate whiskers can be produced under hydrothermal condition using various gypsum precursors,including the gypsum agent,the industrial gypsums (the phosphogypsum,the desulfurized gypsum),and the synthetic gypsums formed by the reaction of calcium oxide or carbide slag with sulfuric acid.Rod-shaped hmemi-hydrate calcium sulfate whiskers with low aspect ratios were formed using the crystalline gypsum agent or impurity-bearing phosphogypsum as the raw materials.Hemi-hydrate calcium sulfate whiskers with high aspect ratios were formed using the desulfurized gypsum and the synthetic gypsums with poor crystallinity and less impurities as raw materials.