A marine sediment core EW17-09 (03¢ª 28¡¯358¡± latitude and 96¢ª 18¡¯788¡± longitude, 870 m water depth, 390 cm core length) was retrieved from the western Sumatra, Simeulue sub-basin, Indonesia. Simeulue sub-basin are situated in eastern Indian Ocean, western part of Aceh Province, which is one of the outer islands in Indonesia. This sub-basin is influenced by adjacent lands in response to tectonic and climate dynamics. The dynamics of marine ecological conditions in the past is an urgent need for providing an analogy to the changes in the future conditions. In this study, the ecological conditions were examined by identifying the vertical distribution of planktonic foraminifera assemblages. This preliminary study demonstrated the presence of Globigerinoides ruber, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, Globigerina calida calida, Globigerinoides elongates, Globigerinoides cyclostomus and Globigerinoides sacculiferus in the samples. The assemblages indicate warm water conditions prevailed in the Simeulue sub-basin during the deposition of the samples. However, subtle ecological changes might have occurred in response to the dynamic of thermocline layer. Cluster analysis of planktonic foraminifera abundance and diversity resulted in three groups showing different ecological conditions. Warm water conditions, high salinity, deeper thermocline with moderate sedimentation disturbance prevailed during the deposition of lower part of the core. Oligotropic water conditions with higher temperature, lower salinity, shallower thermocline layer, and moderate sedimentation disturbance predominated during the deposition of the middle part of the core. The paleooceanography conditions of the upper part of the core are comparable to the lower part. Nevertheless, there are a shoaling of the thermocline in the end of the period. These conditions may indicate an increase in upwelling fluctuations and may represent a change in the IOD-like mean state of the Indian Ocean.