It is vital to understand the stability fields of rock masses of recent engineering structures in Turkey (e.g., high-speed railway tunnel system) before their safely construction. This research project investigates rock mass stability of Ankara-Elmadag tunnel - extending 2975 m, covering 12.5 m diameter space, within a sectional on the high-speed railway system between Ankara and Sivas. Based on our field orientied geological work, we identified two rock formations in the complex geologic setting, proximal to the Ankara ophiolitic Melange. Stratigraphically, Triassic age metasandstone-siltsone (Elmadag formation) was thrusted over the Serpentinites, Cretaceous (Artova formation). In addition to this, rock mechanical parameters based on laboratory studies are received from TCDD (Turkish State Railways). The data derived from these studies are combined to evaluate the rock mass characaterization, quality and strength to determine the engineering behaviours of the rock masses in the region where tunnel will be constructed.