This study aims to investigate the factors affecting the scale of foreign direct investment (FDI) by Korean firms engaged in outward FDI. Among listed companies on the Korea Stock Exchange and KOSDAQ-registered firms, 221 companies conducting FDI in 70 countries as of 1998 were selected as the research sample. The independent variables of interest comprised firm-level factors and tax and environmental factors, totaling 13 variables. The analysis results showed that larger firms tended to increase their FDI. This appears to be because large-scale firms find it difficult to achieve economies of scale solely through the narrow domestic market, leading them to consider direct investment in overseas markets. Additionally, Korean firms' FDI increased in countries with higher tariff rates, indicating that avoiding high tariffs is a major motivation for FDI. Among environmental variables, Korean firms increased their FDI in countries with lower wage levels or larger GDP. This suggests that Korean firms make investment decisions by considering the local market size and cost factors.