High-voltage direct current (HVdc) transmission lines are gaining more attention as an integral part of modern power system networks. Monitoring the dc current is important for metering and development of dynamic line rating control schemes. However, this has been a challenging task and there is a need for wireless sensing methods with high accuracy and dynamic range. Conventional methods require direct contact with the high-voltage conductors and utilize bulky and complex equipment. In this thesis, an ultra high frequency (UHF) radio frequency identification (RFID) based sensor is introduced for monitoring the dc current of an HVdc transmission line. The sensor is comprised of a passive RFID tag with a custom designed antenna, integrated with a Hall effect magnetic field device and an RF power harvesting unit. The dc current is measured by monitoring the dc magnetic field around the conductor using the Hall effect device. The internal memory of the RFID tag is encoded with the magnetic field data. The RFID tag enables remote wireless interrogation using a conventional RFID reader. The advantage of this approach is that the sensor does not require batteries and does not need additional maintenance during its lifetime. This is an important feature in a high voltage environment where any maintenance requires either an outage or special equipment.In this thesis, the detailed design of the RFID tag is presented, including the antenna design and measurements for both the RFID tag and the RF harvesting section, the microcontroller interfacing design and testing, the magnetic field sensor calibration and the RF power harvesting section. The UHF RFID-based magnetic field sensor was fabricated and tested in a laboratory experimental setup. In the experiment, a 40mm diameter aluminum conductor, typically used in 500 kV HVdc transmission lines to carry dc current of up to 1200 A, is used to conduct dc current tests for the fabricated sensor. The sensor was attached to the conductor such that the Hall effect device was 30mm from its surface. A dc current in the range of 100 – 1200A was measured with an accuracy of better than 10% for a reader-to-sensor distance of 1 m.