After large-scale blackout， if certain critical lines fail to operate in the process of restoration scheme owing to the external environment or weak links in the lines themselves， the restoration process will be seriously affected. By referring to the above-mentioned failure， a relative line betweenness is defined employing the concept of betweenness from the complex network theory. Furthermore， a critical degree of lines is put forward to identify the critical lines in the restoration scheme for the transmission network. An analysis is made on the IEEE 30-bus system to identify the critical lines in the restoration scheme. The results show that the index proposed can effectively identify critical lines that are of high availability and markedly influence the restoration result under definite restoration principles. The method proposed is of practical significance in guiding the process of restoration.