Throughout the history Marmara Region has witnessed many destructive earthquakes. Discrimination of fault planes from auxiliary planes with the help of a large number of fault plane solutions provides important contribution to the evaluation of seismotectonic setting of a region. 17 August Izmit and 12 November Duzce 1999 earthquakes are the last devastating earthquakes of the Marmara Region with more than 17.000 deaths. Locating at the end point of the Izmit Earthquake, especially Yalova Region was strongly affected by Izmit Earthquake. In this study, recent earthquake activity of Yalova and Gemlik Regions, which are both located on the western branches of the North Anatolian Fault Zone, are analysed using a new data set from a very dense network. 02.09.2006 is the starting and 31.03.2011 is the last date of completion. During the study 166 source mechanisms were determined in Yalova and Gemlik areas, and most of them have a very good quality.