In these days, it is seen that firms operate not only in their own countries, but also in different geographical areas with the contribution of logistical and financial mobility. This gives the firms the opportunity to survive in their own market and to generate profits from new markets in the increasingly competitive environment that is the result of globalization. On the other hand, there are also the transaction costs that arise out of operating in different countries. In this sense, the relationship between the internationalization and the performance of the firm is being discussed in the literature. The aim of this study is to determine the effect of internationalization levels of the manufacturing firms traded on Istanbul Stock Exchange (BIST) on their financial performances. Using a panel data analysis with the sample of 187 firms for the period of 2008-2015, it is determined that there is a reverse S shaped relationship between internationalization and financial performance. This relationship supports the three-stage theory that suggests firms have achieved internationalization in three stages in the literature, but it shows that the relationship between firm's level of internationalization and firm performance, which is inversely horizontal S form with slope, increases firm performance in the first stages of internationalization, then decreases and then increases again.