Macrofauna communities in Ba Lai estuary, Ben Tre province were investigated in three transects from the river mouth to the dam construction, in the order from the right, middle to the left bank. The community characteristics such as the composition, density, biodiversity, and the distribution pattern were recorded and analyzed. The results showed that the macrofauna communities in the marine section part of Ba Lai river consisted of 76 species belonging to 3 phyla: Mollusca, Arthropoda, and Annelida. In this study, it was notable that a high economic value of Ben Tre Clam (Meretrix lyrata) presented in the Ba Lai estuary with a density of 3160 ind /m2 on the right bank. The highest density was recorded in the mid transect of the river, followed by the right and the left (2907 +/- 4298, 1813 +/- 2056; 1730 +/- 1590 ind /m2, respectively). The biodiversity of macrofauna communities was measured by the species richness, Shannon – Wiener index, and Pielou's evenness. Diversity indices illustrated that the middle bank had the highest biodiversity. However, the statistical analysis results showed that the density and biodiversity indices in these transects were not significantly different. The main reason might be due to Ba Lai dam impact, which has been accreting alluvial, causing these locations gradually being similar in the environmental conditions. The distribution pattern of benthic macrofauna communities in this study should be considered as a typical distribution of benthos in rivers affected by dams. Macrofauna communities which gave rapid responses to environmental changes should be used as a bioindicator.