The formation and transformation of ettringite were studied by measuring the ion concentration in liquid phase and analyzing the composition in solid phase. The effects of C3A, gypsum, lime and C-S-H gel on ettringite formation and transformation were also investigated. The experimental results show that, when gypsum was presented in solid phase, the composition of liquid phase was in favor of ettringite formation. Ettringite formation consisted of three reactions including the formation of [Al(OH)6]3- octahedral, formation of Ca-Al polyhedra prism in which Ca and Al polyhedra arranged alternately, as well as entrance of SO42- into the channel of polyhedra. [Al(OH)6]3- formation, which was the slowest reaction, controlled ettringite formation. The concentration of AlO2- was a main factor that influenced ettringite formation. After gypsum in solid phase was consumed, [SO42-] decreased quickly and [AlO2-] increased, and it was possible that ettringite transformed into hydrated monsulfate calcium aluminate (be called M in short) in thermodymanics. The rate of transformation was controlled by diffusion of AlO2-.