The paper proposes a probabilistic networkflow model in evaluating reliability of electric power transmission systems. The model enables us to thoroughly dissect the structure of power networks

distinguish the elements whose failure can cause cascade outages

and obtain probability to satisfy demands. The examples illustrate the usefulness in analyzing reliability of interconnected power systems. It can be used in both planning and operation security assessments. The shortcomings of the proposed model and future studies along this direction are also discussed.