An experiment was carried out at two selected stations in the Sangu river estuary with the Bay of Bengal, Chittagong, Bangladesh to determine the abundance and monthly variations of phytoplankton and zooplankton communities as well as some physico-chemical parameters of water for a period of twelve months from August 2013 to July 2014. Water temperature, salinity and pH were recorded on the spot during each sampling. The NO3-N and PO4-P of the collected water samples were measured in the laboratory. The highest temperature (28.50 C at station I and 290 C at station II) was in September and the lowest (200 C at station I and 21.20 C at station II) in January. Salinity fluctuated from 13.0 to 32.0 ppt at station I in July and February, respectively and 15.0 to 33.0 ppt at station II in July and February, respectively. The highest pH (8.45) was recorded at station I in October. The highest NO3-N (2.3 mg l-1 and 2.1 mg l-1 at station I and II, respectively) and PO4-P (2.9 mg l-1 and 2.5 mg l-1 at station I and II, respectively) was recorded in August. In total 97 phytoplankton species were identified and 2 types could not be identified. Among the phytoplankton, 58 species belonged to Bacillariophyceae, 14 to Dinophyceae, 11 to Chlorophyceae, 6 to Cyanophyceae, 4 to Polycystinea and 4 to Polyhymenophorea. The highest phytoplankton production (117.3 × 103 cells l-1 at station I and 104.7×103 cells l-1 at station II) was found in August and the lowest (28.77 × 103 cells l-1 at station I and 29.6 × 103 cells l-1 at station II) in January. Phytoplankton bloom was observed in the study area in August. A total of 16 different zooplankton genera were recorded of which 12 belonging to Copepoda, 3 Rotifera and 1 Decapoda. Copepoda was the dominant group followed by Rotifera and Decapoda. The highest number of zooplankton (1,466 cells l-1 at station I and 1,198 cells l-1 at station II) was recorded in January and the lowest (280 cells l-1 at station I and 172 cells l-1 at station II) in August. The highest number of phytoplankton was appeared in August, which may be due to higher level of nutrients and water quality like temperature, salinity and pH were also within the favorable ranges. But zooplankton population was comparatively lower at that period owing to phytoplankton bloom. Therefore, the findings of this study would be helpful for the conservation of plankton community, biodiversity of the waters and congenial environment of the tidal waters.