Calcium sulfate (gypsum) was prepared using both the chemical reagent and the stainless steel refinery sludge from the steel industry. The variables for the experiment were the pH, the reaction temperature, time and the amount of catalyst. The crystal structure and morphology of obtained sample were observed by XRD and SEM, and the thermal property was investigated by DTA. When the amount of NaCl as a catalyst was added 0 ~ 20 wt% to the H₂SO₄-Ca(OH)₂ system, the sequence of product formation was in the direction : gypsum dilrydrate → gypsum hemihydrategypsum dibydrate → gypsum hemihydrate. As a result of DTA, the gypsum hemihydrate was β-type. When using the stainless steel refinery sludge, the crystal structure of samples obtained was the gypsum hemihydrate with β-type as the result of XRD and DTA regardless of the amounts of NaCl while the morphology of the gypsum was thinner and became more needle-shaped with the increasing amount of NaCl.