Determining the price and/or price movements of financial assets such as stocks, bonds and foreign currencies through objective criteria and methods provides various opportunities for individual and institutional investors such as increasing profits or reducing losses. In this study, the reflection of the change in US dollar and Euro currency on BIST 100 index in order to resolve the ambiguity about financial assets within a certain probability by the framework of Bayes Theorem is estimated. In the study, Dollar and Euro exchange rates and BIST 100 index daily closing prices are used as a data covering the period of 2007-2016. As a result of the study, it is found that there is a negative relation between exchange rates and stock exchange index in the short term and a positive relation in the long term. This finding shows that the methods used in determination of the exchange rates such as “Portfolio Balance Approach" in the short term and “Traditional Approach” in the long term are prevalent in Turkey. Besides, it is also found that when Euro exchange rate increases, the BIST 100 index increases with 53,30% probability at the same day and when Dollar exchange rate increases, the BIST 100 index increases with 52,58% probability at the same day