The Afsin-Elbistan lignite deposit, with its 3.4 billion metric tons of reserves, is the biggest lignite basin and one of the most important resources for electrical energy production in Turkey. Kislakoy mining field was selected as the first opencast mine to feed four power station blocks of 300 MW each. Slope instability has been a continuing problem in the Kislakoy opencast mine. Particularly complex failures along a noncircular failure surface appearing at the final slope stage and covering large areas in the mine increase the importance of slope stability. This study outlines the geotechnical characteristics of the lignite-bearing horizons and describes the causes and mechanisms of slope instabilities, which threaten the safety of the mine. Quantitative X-ray diffraction (XRD) analyses were carried out using an interactive data processing system (SIROQUANT(TM)) based on Rietveld interpretation methods. Parametric slope stability analysis and backanalysis were carried out for the failure that occurred at the northwestern final slope stage of the mine. The Spencer-Wright limiting state equilibrium method was used in order to determine with confidence the most representative values of regional shear strength parameters, to explain the failure mechanism, and to assess the conditions at the time of failure. In the analysis, phreatic and piezometric surfaces were considered.