Serious stability problems have led to interruptions in production and advance of mining at Himmetoglu open pit coal mine, NW Turkey. In this study, the causes of the failures are investigated through back analysis on the basis of long-term geotechnical investigations and limiting equilibrium techniques. and the outcomes are presented and discussed. The assessments indicated that failures occurred in the form of multiplanar failure surfaces and the stability of sidewalls was sensitive to the length of the lower part of the basal sliding surface, its strength, and presence of faults. Deterministic analyses suggested a staged-stripping method and the modification of the preliminary design to control stability of the sidewalls at the current pit and in direction of advances, respectively.