The paper presents the results of research concerning the influence of hot deformation parameters on the structure and substructure as well as the plastic properties of a Fe-Ni austenitic alloy. The research was performed on a torsion plastometer in the range of temperatures of 900 divided by 1150 degrees C, at a strain rate 0.1 and 1.0 s(-1). Plastic flow curves have been drawn up and the interrelations have been determined between the process parameters and the recrystallized grain size, inhomogeneity and shape. Functional relations between the Zener-Hollomon parameter and the mean grain size after dynamic recrystallization have been developed and the hot deformation activation energy has been estimated. The examination of substructure on TEM allowed the calculation of structural parameters: the average subgrain area and the mean dislocation density. A detailed investigation has shown that the substructure is inhomogeneous, consists of dense dislocation walls, subgrains and recrystallized regions.