Abstract: Cobalt-based and nickel-based alloy cladding layers were respectively prepared by plasma arc welding on directionally solidified DZ125 alloy surface. Results showed that columnar grains of the cladding layer at the bonding interface between the cladding layer and the substrate were epitaxial. With the decrease of temperature gradient G and the increase of constitutional supercooling degree,the columnar grains became equiaxed grains. During solidification process,the solid-liquid interface moved forward,which resulted in the decrease of temperature gradient. Thus,under the same condition,Ni-based alloy was easier to convert into equiaxed grains than that of Co-based alloy,while in turn less columnar crystals were formed. The microhardness of the cladding layers of Co-based and Ni-based alloy gradually increased from the substrate to their surfaces. Columnar grains of the cladding layer were finer and the hardness were higher,when being compared with those of the substrate. Hardness of equiaxed grains of the cladding layer was higher than that of columnar grains.