Effects of diffusion treatment were investigated on the interface microstructure between a Co-based self-fluxing alloy coating and a mild steel substrate to improve the adhesion strength. Diffusion treatments were carried out at 1373 K to 1418 K for 600 s to 7200 s in an Ar atmosphere. Diffusion treatment improves the metallurgical bonding at the interface due to the diffusion of Co, Cr, W, Ni, and Si from the sprayed coating layer to the substrate and that of Fe and Mn from the substrate to the coating. This mutual diffusion forms a precipitate-free diffusion layer at the interface, and the width of this layer increases in a parabolic manner as temperature and holding time increase. The apparent activation energy for the formation of precipitate-free diffusion layer was evaluated as about 360 kJ/mol. The shearing adhesion strength of the diffusion-treated coating has been remarkably improved to 200 – 400 N/mm2 in proportion to the width of the precipitate-free diffusion layer formed along the interface, although the shearing adhesion strength of the as-sprayed coating was only 30 N/mm2.