Abstract: Diatom analysis and AMS-14 C chronology of peat samples from the valleys in the Dagu Mountain area revealed the history of palaeoenvironment change and sea-level change of that area in Holocene. Comparing with detailed results of coast plains in Japan, a new tectogenesis is adopted to explain the environment change history of the research area. The formation of Dagu Mountain shell-accumulations is believed to be the indicator of the highest sea-level in Holocene.Core sampling was carried out at 11 valleys in the Dayang rivermouth plain in the East Liaoning peninsula, to investigate peat locations as well as palaeoaltitudes. Diatom analysis revealed that the valley peats below 4m altitude were marine facies sediments because marine diatoms dominated, and the sediments above 4m altitude were of terrestrial formation yielding no marine diatom.In order to figure out the distribution and the uppermost altitude of the marine sediment in the valleys, a series of core sampling was conducted at 3 valleys. In the profile of core samplings, the thickness of valley peats decrease from top to mouth of the valleys and finally black clay lamina appears at the mouth, suggesting that the altitude of valley peats descend from the top of valleys, about 4m altitude, to the mouth of valleys. Diatom analysis from the cores LD and DK reveals that the place where marine facies beneath the valley peats were deposited was a transition zone, with the number of marine species decreases while that of freshwater species increases.There is conformity contact between valley peats and the underlain marine facies sediments. The forming of peat indicates such a process that seawater firstly exited which was followed by the growing of plants. As a result, then the bottom of the top layer of valley peat represents the highest sea level. The shell samples for dating were collected from the lower part of the peat near the mouths of valley at the cores LD and DK as well as from marine facies sediments at the coastal plain. The 14 C dating for those shells indicates that the maximum Holocene transgression is around \{6000~\}5500cal.aB.P. , with the altitude of sea-level at around 4m (the average level for high-tide). It is believed to be about 1.7m higher than the current sea-level. Neither the peat layer which is 8m above the sea-level at the valley of Huantukan, nor the shell-accumulations at \{7~\}10m in the west of Dagu Mountain can indicate highest Holocen sea-level because the sediments underlying the valley peats are not of marine facies and the shell-accumulation is formed by shell-tumulus.Evidence for coastal rise such as paleo-coastal erosion and paleo-coatal terrace in Holocene was not observed. Besides, the consistency of the age and the maximum altitude of sea-level in Holocene between the study area and the coastal plains of descending or stable in Japan, Dayang River plain in Dagu Mountain area is not the uplifting area but the descending or stable area in Holocene.