Abstract: 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, however, with reaction time forward, tobermorite was replaced by xonotlite. So far as CaO-SiO₂-Al(OH)₃,-H₂O was concemed, hydrothermal reaction was cdried 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 content was over 15.6%, synthetic mineral greatly changed in structure and haled into hibschite different from 1.1 nm tobermorite. Obviously, xonotlite was not apt to form in the presence of Al³⁺.