During brazing of austenitic stainless steel with copper based brazing material a common failure occurs, namely that the brazing material solutes along grain boundaries, which look like cracks. This unfortunate effect occurred when AISI 304 steel is brazed. To avoid this unwanted effect - since the cracks propagate mainly on high angle grain boundaries - our goal was to enhance the number of special coincident site lattice type grain boundaries with thermomechanical treatment. Experiments were performed for 1, 48 and 72 hour heat treatments at different level of cold rolled materials. After the thermomechanical treatment significant decrease in the crack size was found in depth and width, respectively. The grain boundaries were investigated on electro polished samples in an electron microscope with electron backscattered diffraction technique. The brazing was made with Boehler SG-CuSi3 brazing material.