Abstract: The Bolu tunnel, with a lenght 3250 m is going to be the longest one among the others in Turkish motorway project. The tunnelalignment passes through various lithological unit and zones of strike-slip and thrust faults. The oldest unit of the Bolu tunnel of theAnatolian motorway is the Yedigöller formation of Precambrian age composed mainly ofamphibolite, gneiss, metadiorite andmetoquartzdiorite. Tectonically overlying this unit is the Îkizoluk formation, comprizing ofpyhllite, shale, quartzite and the Çatak limestone member. Upper Cretaceous is represented by a chaotic assemblage, called the Abant complex. Elmalık granitoids have intrudedinto this assemblage in the upper/later. The end of Upper Cretaceous is represented by brecciated marbles and slates of the Atyayla formation. It is overlain by Bayramışlar formation of Paleocene-Lower Eocene, which consists of sandy limestone, calcareoussandstone and breccia. The Findicak formation is represented by sandstone, siltstone, clay stone, marl and limestone. The youngestunits of the alignment of the Bolu tunnel are the Apalar formation of Middle-Upper Eocene, which is represented by alternation ofclay, claystone, clayey limestone, gypsium and marl and the Asarsuyu formation consisting of unconsolidated sediments of Pliocene.These units were piled up within a N-S compressional regime during Late Cretaceous. In Upper Miocene epoch tlie prevailing compressional regime has changed in  which, has created when the North Anatolian Fault Zone, characterized by strike-slip faults.