Abstract: Sancaktepe granite whieh is situated in Koeaeli peninsula to the north of sea of Marmara is known geolo-gically tobe Silurian in age. The purpose of this study is to determine the absolute age of the massif using geoehronologie methods andthe origin of it throungh tine data provided by geology, petrography and geochemistry. The age of the massif was found to be 25+--5 million years by Rb-Sr isochron method. K-A measurements made on the biotites of the granite show that this massif is 254million years old. On the basis of both methods which gave similar results, it could ,be indicated that the granite has onephase of fonnation during Saxonian (Permian) time by post-orogenic movements. This granite is alkaline and rich inquartz was hîghly evolved from the point of view of geochemistry. Initial ratio of Sr is very low, and this proves thatthe granite is deep seated, intrusive and juvenile in character.