Namdapha National Park in Arunachal Pradesh of 1985 Km2 lies in the vicinity of tri-junction of India, Myanmar and China. There is no comprehensive data on the fish fauna and their habitat ecology, especially the fish assemblage of different rivers and streams of the remote areas of Arunachal Pradesh. So the present study will form the basic database for future conservational programme.A total of 58 fish species have been recorded in the present study out of which 29 species are in common with the ZSI record of 1985. While assessing the probable reasons for the decline of fish fauna in Namdapha National Park, following major reasons are noted- 1. A major portion of flat land in the Core area of the Tiger reserve have been encroached by people of Lishu community have an adverse effect in the riverine habitat, as these encroachers have cleared and deforested land in patches resulted in alteration of habitat. 2. Anthropogenic activities have lead to siltation of the natural lakes, which act as nursery for the fish fauna, lead to decrease in fish population. 3. Illegal fishing is also a burning problem for the protection staffs to stop this.