Abstract: There is provided a power transmission line monitoring apparatus installed on a power transmission line to monitor a geomagnetically induced current (GIC) flowing along the power transmission line. The power transmission line monitoring apparatus includes: a main body installed on a power transmission line; a sensing unit embedded in the main body to sense a geomagnetically induced current flowing along the power transmission line; a data collecting and processing unit embedded in the main body and connected to the sensing unit to collect geomagnetically induced current data sensed by the sensing unit and process the collected geomagnetically induced current data; and a wireless communication modem embedded in the main body and connected to the data collecting and processing unit to transmit the data processed by the data collecting and processing unit to a remote data collecting device.