Abstract: Grain boundary penetration behavior occurs easily in the Al/Cu contact reactive brazing.In this paper, the mechanisms of formation and evolution of grain boundary penetration were investigated when contact reactive brazing between 6063 Al Alloy and 1Cr18Ni9Ti stainless steel was conducted using Cu as interlayer.The results show that the grain boundary penetration phenomenon is prominent.Grain boundary penetration depth was up to 200 μm when the brazing temperature was 570℃ and holding time was 60 min.The diffusion of atom into grain boundary was not sufficient but necessary for forming of grain boundary penetration.The key factor to induce grain boundary penetration was non-equilibrium diffusion of atom between the grain boundary and base metal, which led to crystal lattice expanding, and promoted the vacancy transferring into grain boundary, and resulted in a thin groove.And then, micro-cracks were formed in the grain boundaries, the eutectic liquid was sucked into the groove by capillary force, and finally grain boundary penetration was created.The interface reactive layer consisted of Fe-Al intermetallics(IMCs) in the side of 1Cr18Ni9Ti, the adjacent zone was Cu-Al IMCs, welded seam zone was composed of Al-Cu eutectic structure and large blocked Al solid solution.