The study on diversity of fish in the Hau River was continuously implemented during 12 months (July, 2011 to June, 2012) at An Giang (upstream), Can Tho (middle stream) and Soc Trang (downstream). At each area, sampling was conducted at 2 sites (main river and its tributary) using the main fishing gear as trawl net in combination with other available fishing years such as gill nets, fixed nets, lift nets… and at the local fish markets. The results showed that a total of 192 fish species belonging to 16 orders and 49 families was recorded. The most abundant order containing 58 species accounting for 30.2% was Perciformes and the least was Characiformes containing only 1 species. The number of fish species was declining from the upper to lower parts. At the upstream (An Giang), 145 species were recorded followed by Can Tho with 121 species and only 107 species in Soc Trang. The fish species composition was also significantly different from upstream to downstream and between sampling periods. Overall, highest number of species (100-107) was recorded in January, February and April; and lowest number (40) was recorded in July; the remaining months contained 71 to 98 species. At down stream area, highest species number was recorded in March with 48 species and followed by January with 45 species; and lowest number recorded in May with only 24 species among 107 species.