To better understand the complex relationships between palaeoenvironmental change and human activity in the Mekong Delta region of Vietnam during the Oc Eo period (1st millennium AD), some shallow palynological cores were extracted at the archaeological sites of Oc Eo, Nhon Thanh, Go Thanh, Go Thap, Da Noi, Chom Cau, Nen Chua and Canh Den. The results suggest that the flora consisted of tropical plants, with arboreal, herbaceous plants and ferns dominating. Sediment analysis also revealed the influence of marine oscillations in the Mekong River Delta through the distribution of mangrove pollen. A transition from a tidal mangrove environment in the lower layers of the pollen cores, to an alluvial plain environment consisting primarily of marsh plants in the upper part could be identified. Initial occupation during the Oc Eo could also be recognized by the presence of charcoal and pottery, most clearly at 30-40 cm to 1 m depth in the cores. The palynological evidence suggests that communities rapidly moved into the region and established settlements shortly after the local environmental conditions changed from coastal to wetland, as a result of marine regression. This preliminary study provides baseline data for further paleoenvironmental study in this area. Further research is certainly needed to identify the extent of human impacts on the environment in each region at specific times during the Oc Eo period.