Drought is the most complex and damaging event in climatic disasters. Precipitation is the most important factor to control agricultural production in arid and semi-arid regions. The problem of drought in some regions in Turkey shows itself especially effect on the ecosystem and agriculture. The aim of this study is to determine the frequency and severity of drought with Standardized Precipitation Evapotranspiration Index (SPEI) and compare with Standardized Precipitation Index (SPI) using meteorological data measured between 1982 and 2006 in Damlica creek basin located in Istanbul-Buyukcekmece drinking water basin. The SPEI drought index estimates evapotranspiration with Thornthwaite equation. In the regression analysis between the 25-year data set and the annual SPEI and SPI indices, the coefficient of determination (R2) between the annual SPI and SPEI drought indices according to the second order polynomial was found to be 0.977 and found to be statistically significant. However, there were differences between drought severities in monthly, seasonal and 6-month drought periods. According to the annual evaluation results; SPEI Index was determined as 4 years of moderate drought (1983, 1989, 2004, 2006), 7 years of mild drought (1982, 1992, 1993, 1994, 1999, 2000, 2003), and SPI index was found as 2 years of severe drought (1983, 1989), 3 years of moderate drought (1982, 2004, 2006) and 2 years of mild drought (1992, 1993). The SPEI drought index calculated by precipitation, temperature and evapotranspiration data may give healthier results to policy makers in drought mitigation policies in terms of giving more accurate results in agricultural production and drought assessments.