This study aims to observe the reproductive biology of Coilia rebentischii distributed from downstream of the Hau River (one of the branches of the Mekong River) to the estuary of Dinh an and Tran De, in order to supplement the existing scientific data and to serve as a basis for research on artificial reproduction. The study was conducted from July 2019 to June 2020 by collecting fish samples in three sites from downstream of the Hau River and the estuary of Dinh an and Tran De. Results showed that the gonadosomatic index (GSI) of C. rebentischii ranged from 1.10% to 6.92%, which was greater than the fluctuation in the hepatosomatic index (HSI) (from 0.21% to 1.20%); the conditional factor (CF) of the fish also displayed less fluctuation, with the CF reaching its highest value in November. Fecundity of fish ranged from 218–943 eggs/g (mean=495 eggs/g), with a total length of 10.4 cm and total weight of 16.01 g. The maturity size of female C. rebentischii was 11.05 cm, while that of males was 13.69 cm. Result was recorded that breeding season of C. rebentischii starts from January to December, in which the highest breeding is from June to November.