The hydration reaction of a mixture of tricalcium aluminate (C3A) and gypsum with the molar ratio of 1:3 was carried out at room temperature and a water/solid ratio of 4.0. The hydration was carried out in presence of 0, 1 and 3% stearic acid and the mixes were designated as A, B and C, respectively. Ettringite was the only hydration product formed in the presence and absence of stearic acid. Phase composition, microstructure, infra-red analysis as well as degree of hydration were carried out for the different hydration mixtures. The rate of ettringite formation in the presence of 3% stearic acid was accelerated during the first half hour of hydration, and then retardation was occurred. In the presence of 1% stearic acid the ettringite formation was accelerated first till 3 days, then retardation was observed at later hydration ages. The hydration reaction of a mixture of tricalcium aluminate (C3A) and gypsum with the molar ratio of 1: 3 was carried out at room temperature and a water / solid ratio of 4.0. The hydration was carried out in presence of 0, 1 and 3 % stearic acid and the mixes were designated as A, B and C, respectively. Ettringite was the only hydration product formed in the presence and absence of stearic acid. Phase composition, microstructure, infra-red analysis as well as degree of hydration were carried out of the different hydration mixtures. The rate of ettringite formation in the presence of 3% stearic acid was accelerated during the first half hour of hydration, and then retardation was occurred. In the presence of 1% stearic acid the ettringite formation was accelerated first till 3 days, then retardation was observed at later hydration ages.