The difference betWeen the systems of CaO-SiO2-H2O and CaO-SiO2-Al(OH)3-H2O, was studied,especially the effect of Al3+ on the crystallinity of tobermorite was focussed. As a result, in the system of CaO-SiO2-H2O, tobermorite formed early, howevey, with reaction time forward, tobermorite was replaced by xonotlite. So far as CaO-SiO2-Al(OH)3-H2O was concemed, hydrothermal reaction was carried out under the same condition as CaO-SiO2-H2O to study the morphological changes of tobermorite crystals. It was found that Al3+ accelerated the crystal growth of tobermorite to some extent and was in favor of platy crystals. Moreover, with the Al3+ content increasing in the starting material, the morphology of tobermorite did not change magnificently, but platy crystals became more and more eminent. As soon as the Al2O3