The factors that explain changes in population size are a central theme in ecology, and long-term studies of population dynamics are of great interest for life history theory, population ecology, and wildlife management and conservation biology. Endangered Nilgiri tahr (Nilgiritragus hylocrius) is endemic to the Western Ghats Mountains. Studies on the population status of Nilgiri tahr at Mukurthi National Park, Nilgiris, Tamilnadu was conducted for the period 2008 in different seasons (February- March and June -July). The study indicates 424 Nilgiri tahr inhabitants in 17 locations. Males classify as Class I(6+ age,20),Class II( 5+years,18),Class III(2-4 years,39) Nilgiri tahr recorded. Class IV females (2+ years, 161), Class V (1-2 years, 50), Class VI (less than a year, 90) observed during the survey. The highest tahr density present in sector IV (29%), low in sector V (3%) and the enduring sector ranges as of (19 to 27%).the key thing in the present study exposed that 86(21%) Nilgiri tahr is in outside national park. As a result of habitat thrashing, poaching, predation and non accessibility of food, climate change and anthropogenic activities leads the population of tahr is declining.