The energy release rate (ERR) of crack growth as the energy change at thesame time t between the two states of the structure is redefined, one is with crack length aunder the loading a(t) , the other is the state with crack length a + △a under the sameloading condition. Thus the defined energy release rate corresponds to the released energywhen a crack grows from a to a + △a in an infinitesimal time. It is found that under a givenloading history, the ERR is a function of time, and its maximum value should correspondwith the critical state for delamination to propagate. Following William's work, the explicitexpressiors of ERR for DCB experimental configurations to measure the interfacial fracturetoughness have been obtained through the classical beam assumption.