The technology of plasma arc was used to modify the interface adhesion between c hromium coating and steel substrate. The interface microstructure was studied as a function of plasma arc processing parameters. Microstructure analysis was per formed by optical microscopy, scanning electron microscopy and electron probe. T he microhardness distribution along the depth of a cross-section of the chromium coating and the substrate was measured. The results show the energy density of transferred plasma arc is obviously higher than plasma non-transferred arc. The molten interface was obtained by plasma transferred arc. Interfaces between chro mium coating and steel substrate can be divided by plasma non-transferred arc in to three classes: non-molten, a little molten and molten. Good interface bonding was obtained by proper process parameters. The microhardness of chromium coatin g decreases with increasing energy density of plasma arc. The technology of plasma arc was used to modify the interface adhesion between c hromium coating and steel substrate. The interface microstructure was studied as a function of plasma arc processing parameters. Microstructure analysis was per formed by optical microscopy, scanning electron microscopy and electron probe. T he microhardness distribution along the depth of a cross-section of the chromium coating and the substrate was measured. The results show the energy density of transferred plasma arc is obviously higher than plasma non-transferred arc. The molten interface was obtained by plasma transferred arc. Interfaces between chro mium coating and steel substrate can be divided by plasma non-transferred arc in to three classes: non-molten, a little molten and molten. Good interface bonding was obtained by proper process parameters. The microhardness of chromium coatin g decreases with increasing energy density of plasma arc.