Hot deformation behavior of as-cast Mn18Cr18N austenitic stainless steel was studied in the temperature range of 950-1200 ℃ and strain rate range of 0.001-1 s-1 using isothermal hot compression tests. The true stress-strain curves of the steel were characterized by hardening and subsequent softening and varied with temperatures and strain rates. The hot deformation activation energy of the steel was calculated to be 657.4 kJ/mol, which was higher than that of the corresponding wrought steel due to its as-cast coarse columnar grains and heterogeneous structure. Hot processing maps were developed at different plastic strains, which exhibited two domains with peak power dissipation efficiencies at 1150 ℃/0.001 s-1 and 1200 ℃/1 s-1, respectively. The corresponding microstructures were analyzed by optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). It has been confirmed that dynamic recrystallization (DRX) controlled by dislocation slipping and climbing mechanism occurs in the temperature and strain rate range of 1050-1200 ℃ and 0.001-0.01 s-1; And DRX controlled by twinning mechanism occurs in the temperature and strain rate range of 1100-1200 ℃, 0.1-1 s-1. These two DRX domains can serve as the hot working windows of the as-cast steel at lower strain rates and at higher strain rates, respectively. The processing maps at different strains also exhibit that the instability region decreases with increasing strain. The corresponding microstructures and the less tensile ductility in the instability region imply that the flow instability is attributed to flow localization accelerated by a few layers of very fine recrystallized grains along the original grain boundaries.