There are many studies about the calendar anomalies. However, little attention has been given to Islamic (Hijri) calendar effects. Most of studies about Hijri calendar effects are about Ramadan effect. Ramadan effect refers to significantly higher stock returns in month “Ramadan” than the rest of months of the Hijri calendar. This paper investigates the effect of Ramadan on Turkish Stock Market ( BIST 100) using the daily return data for the period of 1988 to 2014 with the application of One Way ANOVA test. The results show that there are significant differences among the Islamic months of years in terms of stock returns, however, there is not a “Ramadan” effect in Borsa Istanbul 100 Index (BIST 100) during the examined period. The month Rajab has the highest and the Rabi’al – thani (Rabi’ II) has the lowest mean returns. Ramadan has the fourth highest mean return.