Accurate flow stress data are essential for the design and optimization of thermo-mechanical processes for a wide range of metallic materials. Hot compression testing is generally used to establish the dependence of flow stress on temperature, strain and strain rate. Flow stress measurements have mostly used cylindrical specimens of different sizes. While the data has been corrected for friction and adiabatic heating, the measurements assume idealized uniform deformation in the test specimen when obtaining the flow stress data from the measured load and displacement. In this paper, it is shown that there is significant non-uniform deformation in the test specimen under practical testing conditions, and a methodology is developed to correct the flow stress data. This correction is generally more significant than the correction due to friction or adiabatic heating and is necessary to obtain more accurate flow stress data. The effect of specimen geometry is also investigated, and a geometry which results in more uniform strain distribution than a cylindrical specimen is used. Measurements are conducted for a prototype powder metallurgy nickel-base superalloy at a low strain rate over a deformation temperature range. Constitutive equations are constructed with the measured flow stress data. Finite element modeling of the tests using the corrected data provides a better agreement with the measured loads. The grain microstructures vary with the test parameters and are used to correlate the flow behavior.