The high-resolution study of the stratigraphic sequence of Ostia basin is based on multidisciplinary geoarchaeological study that integrate 17 radiocarbon dates obtained on 12m of stratigraphy. Three steps can be distinguished: pre-harbour, harbour and post-harbour sequence. (i) The pre-harbour step is feature by a fluvial-marine environment dated between the 9th and 8th BC. (ii) The base of the harbour sequence is characterized by a facies of dark grey silty clay; it is a quiet area in connection with both the river and the sea. It starts at the earliest in the middle of the 4th c. BC (360 BC) and is attested until no later than the end of the 3rd c. BC (205 BC). At that time the depth of the basin reached 6m below zero antique marine biological. This unusual depth is very significant for the time and implies that seagoing vessels could access the basin. The sedimentation rate is high: 2, 6cm/ year. (iii) The summit of the harbour sequence is marked by yellow sands brought by floods of the Tiber. The protective role of the basin decreases between 165 BC and 5 AD and depth is no longer than 2.5m under the ancient marine zero. Most ocean-going ships cannot access it anymore. The sedimentation rate slowed to 0.8 cm/ year.(iv) The basin loses its function at the beginning of the imperial period because its depth was only 60 cm between 45BC and 25 AD and about 30cm between 80 and 230 AD. Three hiatus are observed in the stratigraphy: (h1) at -6m corresponds the digging of the basin. (h2) at -2.5m probably represents a dredging. (h3) at -0.50m corresponds a decrease in the sedimentation rate or traduce the maintenance of a structure related to a function modification of the sector.In order to compare the depths of the harbour basin of Ostia and the ancient Tiber a core sample was carried out on the southern shore of Isola Sacra. It provides a stratigraphic sequence of 11m and reveals the presence of the ancient mouth of the Tiber, dated between the 5th and 3th c. BC. A depth of 6.7m below the ancient marine biological zero is attested. It is clear that the depths of the Tiber and the harbour basin are very close. Does the basin has been dug by fitting the Tiber bed or else the mouth channel was dredged to work with the basin?