The aim of this paper is to examine the effects of mandatory IFRS adoption on foreign portfolio investments and firm investment efficiency. After analyzing these aspects of Taiwanese publicly traded firms during pre-IFRS (2009 and 2011) and post-IFRS (2013 to 2015), the empirical evidence shows that IFRS adoption has a greater impact with the higher foreign shareholdings and lower under-investments. Overall, these findings are consistent with the notion that mandatory IFRS adoption facilitates cross-border capital flow and highlight the importance of considering the change of investment efficiency when examining the economic consequences of mandatory IFRS adoption.