Abstract: The bond interfacial structure of fly plate and its influence on the hardness of explosive welding 304/Q245R composite plate after stress relief annealing were investigated by means of scanning electron microscope, X-ray diffractometer and nano indentation etc. The white-layer zone was constituted by nano-sized austenitic and martensitic grains, which formed by dynamic recrystallized. The fiber zone was composed by tempestuously elongated strips austenitic grains and martensitic grains which located on twin intersections. The fiber zone was composed by tempestuously elongated strips austenitic grains and martensitic grains which located on twin intersections. The deformated zone was composed by tempestuously torsional austenitic grains and a small amount of martensite grains which located on twin intersections. The nano hardness of white-layer zone, fiber zone and deformated zone increased by 32%, 13.5% and 9.1% respectively compared with initial austenite zone of fly plate. The nanocrystalline strengthening of the whitelayer zone and compared with the deformation zone, the martensite phase transformation strengthening of the fiber zone which is larger were the main reasons.