Abstract: The WNW-ESE-trending Eskişehir fault zone which separates the Aegean-western Anatolian block from the central Anatolian block is a right lateral strike-slip fault zone with a normal component. The fault zone is characterized by fault segments which trend between E-W and NW-SE around Eskişehir. Syndepositional and post depositional faults cutting Pleistocene and Holocene units indicate that the Eskişehir fault zone has been active since at least Pleistocene. At least 14 earthquakes (M 4)occurred on the Eskişehir fault zone in the 20th century and the 20th February 1956 Eskişehir earthquake (M=6.4) was the largest event in this century. The isoseismal map of the 1956 earthquake shows that this earthquake occurred on about 10-kmlong WNW-ESE-trending Oklubal-Turgutlar segment. There is no major earthquake record in the historical earthquake catalogues before the 20th century. However, both existence of mud dykes in Pleistocene units and deformation of Holocene deposits in front of fault scarps indicate that fault segments are active in this area and several M 6 earthquakes occurred in the last 10 000 years. Fault plane solution of the 1956 earthquake and field observations indicate that the Eskişehir fault zone which played an important role in the development of Eskişehir and İnönü plains is a transtensional fault zone.