Abstract: İzmit Bay, which is the eastern continuation of The Sea of Marmara, ending in three successively smaller bays, is about 50 kmlong and 1.5 to 10 km wide. It is a part of an east-west trending graben, which has been shaped by the NAF-Zone system. The Bayfloor is a shallow-shelf area.This study is based on the re-evaluation of the results from 13 seismic profiles. The recent sediments at The Bay floor display apre-Pliocene/Pliocene, Middle & Late Pleistocene andHolocene succession from bottom to top, and since, it can be defined as a PlioQuaternary Unit. Thickness ranges from 10 to 50+ msecs. Sediments are mainly composed of terrigenous elastics, varying from clayto pebble-gravel in grain size, and at a lesser amount, carbonates. Young cover is thicker at the south and east than other parts, because of deltaic pro gradation.The alluvial fan in the south, that has been forming Dil Burnu area, was deposited as Yalakdere Deltaic Sequence. Young sediments deepen eastward, due to a NNW-SSE trending vertical movement at the east of Dil Burnu. This structural line has been producing a submarine fan, off Dil İskelesi to the north. As a result of this lineation, it can be concluded that, there is a local tilting in thefan area.Since the faults at depths of The Bay bifurcates toward the surface, the development of flower structures must be emphasized.The Bay, itself, is probably a locked-up basin. As being examplified by the area between The Bay and Lake Sapanca, the easternmost (innermost) basin will may be a lake grodually passing (in)to a marshy area, through the filling-up by sediments.