One of the most seismically active countries in the world is Turkey surrounded by the African, Arabian, and Eurasian plates that control seismotectonics of Turkey. The North Anatolian fault, which lies from Erzincan to Aegean Sea (about 1500 km long), produces large earthquakes frequently. One of the characteristics of this fault is earthquake migration along the fault which began in 1939 (the Erzincan earthquake) in the east toward the west of the country. The last earthquake on this fault occurred in 1999 and the next large earthquake is expected to occur around Istanbul that is the biggest city in Turkey. Studying about the next earthquake on the fault is very difficult because unbroken part of the fault is beneath Marmara Sea. Shear-wave splitting is one of the powerful tools on active fault observation studies. For this purpose we observed shear-wave splitting in Marmara Sea, but because of the sparse station distribution of the Boğaziçi University, Kandilli Observatory and Earthquake Research Institute (BU-KOERI) network we only detect S-wave splitting in the western part of the sea. We found out that the LSPD (leading shear-wave polarization direction) is parallel with the strike of the Ganos fault, which is the major fault of the study area and produced the Ms=7.3 earthquake in 1912. One of the possibilities for the cause of this anisotropy is that fault healing has not finished, and that split waves polarization direction is parallel to the fault strike owing to microcracks produced by the 1912 earthquake. Our result is consistent with the historical records that there has been no large earthquake since 1912. Shear-wave splitting method is very useful to investigate highly active fault zones, and should be applied as soon as possible in and around Marmara Sea with dense seismic network and sufficient data.