A potential energy framework for assessment of grid vulnerability was presented. In the framework, the branch potential energy function model was constructed. Two indexes, current vulnerability and forecasting vulnerability, were calculated. The current vulnerability was used to identify the current vulnerable area through calculating the distance between the current transmitted power and initial transmitted power; and the forecast vulnerability under variation of power injection was used to predict the vulnerable area of next step and verify the current vulnerable area. Numerical simulation was performed under variant operating conditions with IEEE-30 bus system, which shows that almost area of 90% overlaps between current vulnerable area and forecasting vulnerable area, the overlapped area is termed as inherent vulnerable area of grid. When considering N-l contingency, the assessment results of this method proposed agree with those of optimal power flow. When considering N-2 contingency, optimal power flow fails to obtain correct results, while the method based on energy framework gives reliable results.