Since the late 1980s, as the internationalization of Korean firms has progressed, foreign direct investment (FDI) has also become increasingly active. Common methods of corporate FDI include greenfield investment, joint ventures, and mergers and acquisitions (M&A), each of which entails significant differences in the risks borne by the investing firm and the coordination capacity of the parent company. For firms in countries like Korea that have not yet established the systematic form of multinational enterprises, the question of which type of investment method to select as a mode of overseas expansion is critically important. This study examines the characteristics and development of Korean firms' FDI over the past decade and, under the assumption of stock market efficiency, measures changes in firm value for each investment type. The analysis reveals that joint ventures constituted the majority of Korean firms' overseas expansion, while M&A-based entries remained relatively rare. Regarding stock market reactions, overseas M&A cases exhibited negative cumulative abnormal returns, confirming that FDI through M&A had a negative impact on firm value, whereas no statistically significant abnormal returns were observed for joint ventures or greenfield investments. These results can be interpreted as evidence of the problems and difficulties associated with overseas M&A compared to joint ventures and greenfield investments from the perspective of firms in the early stages of international investment.