The objective of the study was to determine the composition and density of zooplankton in order to assess natural food source in the rotational shrimp-rice system. The study was carried out from October 2013 to July 2014 at Hong Dan district, Bac Lieu province. Zooplankton samples were collected at three sites such as water inlet and two rice-shrimp systems stocked at 3 shrimp.m^-2 density. A total of 83 species of zooplankton was recorded belonging to 5 groups, in which Rotifera was the highest diversity group with 46 species (55%). Species number of rice crop (63 species) was higher than that of shrimp crop (51 species). Zooplankton density of the rice and shrimp crops was varying from 68,250-771,375 ind.m^-2 and 24,643-1,526.750 ind.m^-2, respectively. Zooplankton density was recorded quite high (167,063-298,056 ind.m^-2) at stage before stocking shrimp supplying a natural food source for shrimp larvae. Common species were found such as Tintinnidium fluviatile, Brachionus angularis, Brachionus plicatillis, Polyarthra vulgaris, Synchaeta sp., Eucyclops serrulatus, Mesocyclop leuckarti, Nauplius of Copepoda, Polychaeta and Bivalvia larvae. Mean Shannon-Weaver diversity index (H’) of rice crop (2.40.19) is higher than that of shrimp crop (1.20.61). In general, zooplankton composition in the rotational rice-tiger shrimp was relatively diverse provided the useful natural food for shrimp. These results contributed to increase the survival as well as the productivity of shrimp at the crop end.