Fish diversity of Haraz River Was Studied from November 1999 To August 2000. This river is situated in The Southern Caspian basin of Iranian Mazandaran Province. Five Stations Were Selected along The River. 522 specimens were Collected and studied, that belonged to 20 species and 9 families: Petromyzontidae, Cyprinidae, Cobitidae, Salmonidae, Gobiidae, Mugilidae,Balitoridae,Poeciliidae and Gasterosteidae.Cyprinids comprised 67.2 percent of the species. Seventy percent of species were resident and 30 percent anadromous. Rain bow trout (Oncorhynchus mykiss), Gold fish (Carassius auratus), (Liza saliens) and Stickle back (Gasterosteus aculeatus) and Gambusia (Gambusia holbrooki) were introduced species. Shannon- Wiener's coefficient of diversity (H'), Food selectivity and Horn's coefficient of niche overlap were estimated.