The effect of initial alloy state on hot deformation and dynamic recrystallization (DRX) of GH984G alloy, used to manufacture large scale forgings for 700 °C advanced ultra-supercritical (A-USC) coal-fired power plants, was studied by isothermal compression test. The experimental results show that the apparent activation energy of hot deformation for the as-cast alloy is larger than that of the hot-rolled alloy. The reasonable ranges of processing parameters for the as-cast and hot-rolled alloys are 1100–1200 °C/0.01–0.5 s−1 and 1100–1200 °C/1.0–10 s−1, respectively, based on the processing maps. The flow instability domain of the as-cast alloy is larger than that of the hot-rolled alloy. The different hot deformation behaviors of the as-cast and hot-rolled alloys result from the various initial grain sizes and DRX mechanisms. The main DRX nucleation mode of the as-cast alloy with large grains is continuous dynamic recrystallization (CDRX). However, for the hot-rolled alloy with fine grains, the discontinuous dynamic recrystallization (DDRX) is dominant.