In order to investigate the intergranular reaction mechanism during sintering of molybdenum-copper powders with submicron components, a Mo-15Cu composite with an intergranular reaction layer was prepared by a submicron-activated layer method. The histomorphology, connectivity and structural characteristics of Mo-15Cu alloy were characterized by X-ray diffracto-meter (XRD), scanning electron microscope (SEM), energy dispersive spectrometer (EDS), transmission electron microscope (TEM), and high-resolution transmission electron microscope (HRTEM). The effect of submicron-activated layers on the intergranular reaction of Mo-15Cu composites was investigated. The results show that the submicron-activated layer with a thickness of 0.5 μm is formed by adding 8wt% submicron powder. The submicron-sized Mo particles change the intergranular reaction mechanism through flow and diffusion, forming an intergranular reaction layer with a thickness of about 5 nm and achieving metallurgical bonding.