Ettringite is used to improve the early strength of concrete or to make shrinking compensate concrete by using it's expansion characteristic. But the formation of delayed ettringite(DEF) results in the expansion break in concrete. This is the duality of ettringite action in concrete. In order to expose the mechanism of the duality of ettingite action in concrete, several aspects, including ettringite's formation condition and ettringite's stability, were discussed in this paper. This is helpful to make a suitable use of expansion characteristic, and to prevent from some unfavorable effects.