About 37% of South Korea’s greenhouse gas emission is from electricity generation. Most of the country’s electric power is fundamentally generated by nuclear, thermal and LNG facilities. And LNG, of them, is characterized to require high cost for power generation but CO₂ coefficient is lower than thermal generation. Amid the ongoing global efforts to tackle global warming, shale gas introduction and changing global environment, LNG prices are expected to fluctuate. Against this backdrop, this paper seeks to perform scenario tests on LNG fuel cost fluctuation and examine its long-term effects on generation expansion planning.