South Korea and China established their diplomatic relations in 1992. The relation has always been positive, despite their different ideology, even better during Park Geun Hye and Xi Jinping presidencies since 2013, from having the same historical enemy. The two leaders wanted to make East Asia become peaceful and worried-free. Yet, North Korea has always been a main topic in the talks between ROK-PRC on security, from the weapon of mass destruction. During Park's early period, she decided to tilt toward China, for trading, and moved away from US, but still kept relation at the arm-length. However, from mid-2015, the relationship started to show friction as South Korea accepted U.S.' defensive weapon, THAAD. The act showed that South Korea did not trust China, despite their cooperation on economy that they decided to accept U.S.' security shield. Park's decision was not based on security dimension alone, as economic benefit also gives a big impact, for going back to traditional ally. This paper aims to find Park's reason for accepting THAAD, despite knowing it would hurt economic relation with China, which is not based on security reason alone, as economic benefit also plays a big part on changing Park's decision.