The increasing demand for electrical energy and the limitations of existing transmission infrastructure have made transmission capacity enhancement an important objective in modern power systems. This study investigates the enhancement of Surge Impedance Loading (SIL) as a metric for transmission line capacity improvement through the application of a Unified Power Flow controller (UPFC) on selected weak transmission corridors of the Nigerian 33o kV power network. SIL was adopted as the principal performance index because it represents the natural loading capability of a transmission line under balanced reactive power conditions. A mathematical formulation of SIL was developed from the transmission line surge impedance relationship, while the UPFC was modeled as a combined shunt-series Flexible AC Transmission System (FACTS) controller capable of voltage regulation, reactive power compensation, and power flow control. Comparative simulations were carried out for transmission lines with initial SIL values below 382.1o53 MW under uncompensated and UPFC-Compensated conditions. The results showed that the total SIL of the studied network increased from 4274.427 MW to 4448.37o MW after UPFC integration, corresponding to an overall improvement of 4.o694%. Significant line-by-line enhancements were also recorded on major corridors such as Onitsha/New Haven, Onitsha/Alaoji, New Haven/Ugwuaji, Ikot Ekpene/Ugwuaji, Makurdi/Jos, and Ugwuaji/Makurdi. Several lines attained or exceeded the benchmark SIL threshold following compensation. The findings demonstrate that the UPFC is highly effective for improving transmission loadability, voltage stability, and utilization of existing grid assets. The study concludes that UPFC deployment offers a practical and economically attractive approach for strengthening the Nigerian transmission system and meeting future electricity demand without immediate largescale network expansion.