We have investigated the diffusion, agglomeration, and reaction at Cu/3C-SiC interface, using soft X-ray emission spectroscopy (SXES) and photoemission electron microscopy (PEEM). By measuring Si Kβ emission, the diffusion and/or agglomeration of Cu atoms at Cu (60nm)/3C-SiC interface were found to occur obviously after annealing at 350 o C and to be enhanced with increasing annealing temperature. PEEM observations for a Cu (30nm)/3C-SiC sample indicated that the Cu film began agglomerating at 550 o C annealing from the edge of the Cu film on the 3C-SiC substrate. The agglomeration spread from the edge to the middle region of the Cu film with the increase of temperature. At 850 o C annealing, the Cu agglomeration was observed in the whole area of the Cu film. The formation of Cu 3 Si was found from SXES analysis for Cu/3C-SiC(001) specimens annealed at temperatures higher than ~850 o C, which indicates the agglomerated material observed by PEEM at 850 o C is Cu 3 Si.