This research aimed at showing the effect of foreign portfolio investment (FPI), both in buying of shares or sale of shares by foreign investors, inflation and gross domestic product on the market capitalization in the Amman Stock Exchange for the period 2005-2016. The financial data were collected through the official website of the Amman Stock Exchange and the reports issued by the Central Bank of Jordan. To achieve the purpose of the study, the financial data were analyzed utilizing statistical methods offered by SPSS package. The study concluded that there is a statistically significant effect on both the purchases and sales by foreign investors on market capitalization. The study also found no statistically significant effect between inflation and market capitalization. Key Words: FPI, Portfolio, Investment, Amman Stock Exchange.