The thermodynamical model of thaumasite formation in cement based materials was established for the direct route and woodfordite route. Data of the thermodynamical model indicated that thaumasite can be formed between 0℃ and 25℃ by direct route while woodfordite, instead of pure thaumasite, can be formed from ettringite, C S H, calcite and water at 5℃, and woodfordite formed more easily than thaumasite by the direct route. Enthalpy change of the direct route of thaumasite formation showed that the direct reaction is an endothermic reaction, and equilibrium constant of the direct reaction decreases as reaction temperature increases. It was also indicated that low temperature is favourable to thaumasite formation.