A new concept of flow betweenness of power transmission line is proposed as a criterion for critical line identification in a power system. This new concept is based on comprehensive consideration of the extent of a transmission line used by different generator-load pairs

and the influence of generating capacity and load level. The criterion can adapt to the variation of power flow distribution under different operation modes. It can also quantify the contribution and effects of transmission line on the whole grid power transfer with a more comprehensible physical background. Case studies based on Gansu power grid show that the flow betweenness distribution of transmission lines has typical heterogeneous characteristics

i.e. a small number of transmission lines play critical role in the power exchange， and the load loss under cascading failure based on flow betweenness is significantly higher than the existed electrical betweenness and random cascading failure manner. Moreover

the critical line identification results and the influence of different operation modes on flow betweenness are closely in line with the actual operation characteristics of Gansu power grid. The case study results clearly validated the effectiveness of flow betweenness.