Pakistan managed extensive canal irrigation systems and has 19 Ramsar sites. Almost 27,977 fish species are present globally; there are 786 marine and 171 freshwater fishes in Pakistan. Fish diversity and distribution are closely related to various factors i.e. depth of water, availability of food, topography, breeding sites, and physicochemical features. Therefore, it is necessary to study the diversity of ichthyofauna in various regions and elevations. So, this study was planned to examine the fish diversity of Chashma Barrage to include more data related to fish diversity and distribution. Data were collected both by direct (physical count) and indirect methods (meeting with local people of Chashma Barrage) to find out fish diversity of Chashma Reservoir, Mianwali, from April 2016 to March 2017. In the current study, 1089 fish specimens of 21 species were documented from Chashma Barrage monthly. The Shannon-wiener Index (H') was recorded as 2.17, Simpson Index (S) was 0.16, Richness (R) was 20.67 and Evenness (E) was 0.712. It is concluded that study area has high diversity of fishes. Chashma Barrage is an important wetland for fishes, it should be declared as Ramsar site.