Datalog with negation(called Datalog～ )is an extension of Datalog.The fixpoint characteristic of well -founded model semantics of Datalog～ is discussed in this paper.At first,the stability transformation is revisited, and a new operator OP and its fixpoint are defined.Based on this,the oscillating fixpoint model semantics is defined.Then,it is shown that for any datalog～ program the oscillating fixpoint model is identical to its well-founded model,and so,the former can be viewed as a constructive definition of the latter.