Oita coastal plain is situated in the east-central part of Kyushu, facing Beppu Bay, the westernmost part of Seto Inland Sea. The plain mainly consists of alluvial plains of the Oita, the Ono and the other small rivers.<br> In this paper, Holocene deposits and geomorphic development of the western part of Oita plain are discussed on the basis of drilling data, carbon-14 dates and field survey.<br> The main results obtained in this paper are as follows:<br> 1) Under the plain there are higher and lower terraces and valley bottoms buried with Holocene deposits. The buried lower terrace is 5-40 meters below present sea level and gradient of the surface is steeper than that of the present Oita river: 4.4 permillage toward downstream. The longitudinal profile along the Oita river indicates that it is correlated with Lower Yokcse surface which was formed during the latest glacial age. During the maximum stage cf the latest glacial age, the buried valley was formed as the paleo Oita river. The drilling data obtained in Oita plain suggested that the deepest part of the valley floor lies 39.1 meters below sea level. In the early Holocene, lower terrace and valley floor were drowned by transgression and were buried with sediments. With progress of transgression, buried higher terrace was formed as the abrasion platform.<br> 2) The Holocene deposits are chiefly composed of sand and silt. At the midtown area of Oita city, Holocene deposits exceeds 47 meters thick, and at the reclaimed land along the coast, the thickness of the deposits are 32 meters or more. The middle mud layer yields large quantities of fossil shells e.g. Anodontia stearnsiana, Fulvia hungerfordi, Cyclina snensis etc. which indicate an inner-bay muddy and sandy bottom at that time. At the midtown area of the city, the middle mud layer occurred between 6, 250 and 3, 460 years B. P. At the locality 6 kilometers upstream from the river mouth, Crassostrea gigas is frequently found in the mud layer. The marine mud layer ranges in height from 1.5 meters to 2 meters above mean sea level. It is determined that the oystershells in upper and lower part of the layer have carbon-14 ages of 6, 600 and 7, 100 years B.P., respectively.<br> 3) At the western part of Oita plain, Holocene transgression reaches the maximum stage in the period between 6, 250 and 6, 600 years B. P. Field evidences show that the highest sea level of this transgression is between 2 and 3.6 meters above present mean sea level.<br> 4) In the northern part of the city, there is a remarkable change in elevation of the lower sand and gravel layer and of “Kikai-Akahoya” volcanic ash bed. This buried scarp probably corresponds to the north-thrown fault named “Nangan (South coast) Fault” by T. Shuto and M. Hitaka (1971). However, this is able to regard as the dissected valley wall during the maximum stage of the latest glacial age, since it does not continue further east, and the average slip rate of faulting is extremely high assuming that this is of fault origin.