Based on flow betweenness, transmission betweenness puts into consideration the role of a line in transmission of reactive power. Transmission betweenness is proposed and applied into identification of critical lines in power grid. Since abundant reactive power support is the base of voltage stability, transmission betweenness considering the effect of reactive power flow can identify critical lines that are important to voltage stability. Successive faults of critical lines can destroy transmission corridors of active and reactive power flow. On this occasion, some loads have to be cut off to maintain frequency and voltage stability. So, load shedding based on optimal power flow is calculated as the evaluation index to verify that transmission lines with high transmission betweenness are critical indeed in power grid. The results of IEEE 39-bus system and IEEE 118-bus system show that transmission betweenness is superior to flow betweenness in identification of critical lines that influence voltage stability, and the identification method of critical lines in power grid based on transmission betweenness is reasonable and effective. This work is supported by the National Key Research and Development Program of China (No. 2016YFB0900100), and Science Technology Program of State Grid Zhejiang Electric Power Company (No. 5211SX1500R4).