Diversity, relative abundance and some aspects of the biology offishes below the Tisisat Fall of Blue Nile River was studied from October 2010 to March, 2011 using gillnets of stretched mesh size 6, 8, 10 and I2cm, monofilaments of different mesh size, castnet and hook and line. Eight hundred fifty seven fish specimens were collected during the study period in both dry and wet seasons. Five family, six genera and eight species were recorded from the Blue Nile River below the fall. Family Cyprinidae was best represented in this river. The fish species compositions of the Blue Nile River below Tisisat Fall were different from above the fall. Wotetomider (Wm) had the highest diversity (H'= 1.66) followed by Abenaze (Ab, H'= 1.64) and Sefania ( Se, H'=I.58) sites in wet season where as Se had highest diversity (H'= 1.44) followed by Ab ( H'=I.33) and Wm (H'=I.23) during dry season. Labeobarbus intermedius; Labeo forskalii and Mormyrus kannume with percentage IRl of 51. 9 and, 29.14 and 5.99 respectively were the most dominant species. The length weight relationship of L. intermedius, L. forskalii and M. kannume were curvilinear and the relation was statistically significant (P<O.OOI). The mean Fulton Condition factor of L. intermedius and M kannume was statistically insignificant (P>0.05) in season and sex but that of L. forskalii was statically significant (P< 0.01) in seasons only. The sex ratios of L. intermedius, L forskalii, Labeobarbus crassibarbis and Bagrus docmak were different from 1:I and were statistically significant with P<0.05. Absolute fecundity of L. intermedius was linearly related to total length, total weight and gonad weight. There was fishing activity in the study area but there are many problems raised by the local farmers. Some of the problems were lack of extension service, absence of appropriate fishing materials and lack of market and post harvest processing technologies. To sustain the biodiversity, the volume of water coming out from the source should be maintained at an appropriate level. Key words: biodiversity, species composition, reproduction, sex ratio, relative condition, fecundity