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 concerned, 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 content was over 15.6%, synthetic mineral greatly changed in structure and turned into hibschite different from 1.1 nm tobermorite. Obviously, xonotlite was not apt to form in the presence of Al3+.