This thesis examines the trends, patterns and determinants of Japan's foreign direct investment (FDI) in Thailand during the period 1970 to 2003, taking into account the up to date literature and most recent data. The research consists of analyses of the trends and patterns of Japan's FDI in Thailand, literature reviews of the theories and empirical studies of FDI in general and in particular of FDI in Thailand, and an econometric analysis of the determinants of Japan's total FDI in Thailand and Japan's FDI in Thailand's manufacturing and services sectors.