AISI 304 type austenitic stainless steel samples were subjected to different thermomechanical treatments in order to investigate the effect of thermo-mechanical treatment on the grain boundary structure of the material. Electron back scatter diffraction measurements have been carried out in order to obtain information about the boundaries of the treated specimen. The measurements showed that achieving the same deformation with the same number of deformation cycles and same heat treatment temperature, the application of shorter heat treatment holding time was advantageous in aspect of grain boundary structure comparing to the thermo-mechanical treatments with longer holding time. The frequency of the special Σ3n type CSL-boundaries excluding coherent twin boundaries was significantly decreased by increasing the heat treatment holding time of the samples from the very short heat treatment periods. Extending the holding time further, the frequency of the special Σ3n type CSL-boundaries excluding coherent twin boundaries increased and reached the results applying the shorter heat treatment periods.