The study was carried out to assess the fishing gears used, their catch composition and their effect on fish diversity in Dekar haor of Sunamganj district from September 2015 to August 2016. A total of 20 types of fishing gears under nets, traps, hooks and lines, wounding gear and fish aggregating devices were recorded during study period. Considering fishing nets the highest and the lowest CPUE (kg gear1day-1) were 4.67 (ber jal) and 1.56 (thela jal), respectively. Of the traps, the highest and the lowest catch per unit effort (CPUE) were 2.3 (borshi) and 0.09 (plastic chai), respectively. It was observed that huge amount of fishes were caught by seine net (ber jal) and gill net (current jal), and the rare species are damaged through this bulk catches. Seine net and gill nets are the most detrimental gears compared to others. A total of 51 fish species belonging to 8 orders under 20 families and 32 genera were recorded. Twenty one (21) species out of 51 were found threatened. Among the threatened fishes, 8 species were vulnerable (VU), 10 endangered (EN) and 3 critically endangered (CR). Abundant fish species were Puntius ticto (titputi), Mystus tengara (tengra), Amblypharyngodon mola (mola), Mastacembelus armatus (baim), Macrognathus pancalus (guchi baim), Nandus nandus (bheda), Glossogobius giuris (baila), Macrobrachium malcolmsonii (ichha), etc. Order-wise catch composition shows that Cypriniformes (33%) was the most dominant order followed by Perciformes (25%), Siluriformes (22%) and others. It is observed that availability of fishes in the haor is gradually reducing every year due to different causes. If it continues in this way, haor will be devoid of fishes and other organisms very soon. Therefore, the government should immediately take appropriate initiatives to protect the biodiversity and ecosystem of the haor for the greater interest of the nation.