The reliability plays a significant role in power systems and it is an important objective or constraint intransmission expansion planning. Firstly, a DC optimization model was proposed to calculate the maximum arrivalpower at each load point. Compared to the network flow method, DC model is closer to the actual power flow and itis able to obtain more realistic reliability assessment results.Furthermore, a novel sensitivity index (SI) was also pro-posed to choose the most effective line so as to enhance thenodal and/or system reliability. The Monte Carlo simulationis used to simulate the system components state. This improved reliability evaluation method and SI can be used for transmission expansion planning or maintenance scheduling. Tests are performed using 6-bus system derived from the Garver’s system and the IEEE 10-machine 39-bus sys-tem. The results show the effectiveness of the method.