A finite element model was developed for isothermal forging of a nickel-base superalloy, Waspaloy, using DEFORM® simulation software. The effects of the forging temperature in the 980 to 1140 °C range, strain rate in the 0.01–1 s−1 range, and true strain up to 0.83, on the hot deformation behavior and microstructure evolution were examined. The model was validated by comparing the predictions with the experimentally measured true stress–true strain curves and the average grain sizes. Reasonable agreement was obtained between the predictions and experimental data. The modeling results show that under investigated conditions the distributions of hot deformation parameters, i.e., strain, strain rate, and temperature, were inhomogeneous within the forging. The maximum strain, strain rate, and temperature were mainly observed at the edge and center of the forging. The inhomogeneous deformation resulted in the development of the dead metal zones close to the dies and an inhomogeneous microstructure, i.e., inhomogeneous distribution of dynamic recrystallization (DRX) volume fraction and average grain size, in the forging.