In this study, uncertainties in isentropic and exergetic efficiency of a High Pressure Turbine (HPT) of large-scale lignite fired power plant have been analyzed for five different design power outputs (100, 85, 80, 60 and 40%) and by using one real data of the plant. Primarily, the differential method, which depends on the sensitivities of pressure and temperature, was used in carrying out this study. In addition, extreme case analysis was conducted in order to determine the boundaries of the uncertainties for the rare case occurrences. The range of uncertainties for isentropic and exergetic efficiency of a HPT were found to vary between 3.35%-2.62% and 2.55%-1.50% for a power output level of 144-360 MW, respectively. The differences in percentage between the possible maximum and minimum extreme values were calculated as 11.6% for isentropic efficiency and 6.6% for exergetic efficiency. It is concluded that the dependency of exergetic efficiency uncertainty is more sensitive to pressure whereas it is less sensitive to temperature variations. Copyright (C) 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.