The aim of this study is to investigate the Day of the Week Effect (DWE) in Borsa Istanbul BIST-100 Index. For this purpose, a dataset of closing prices of the firms was gathered from January 03.2005 to November 06.2015. The data were transformed to return series by taking logarithmic differences, and analyzed with GARCH (1,1) Model. According to the findings, although the coefficients representing the returns of Monday and Thursday are statistically significant, the returns of the trading days of the week are equal. Consequently, for the related period, DWE was not detected in BIST-100 Index.