In the article it have been expounded that laser build -up welding had been done on the 45 steel by used high power CO2 laser. The effect to microstructure and rigidity that have been done by the build- up welding technique altered was studied . In the experimentation condition, we find the build-up welding layer which micro-structure is high density and dispersion metal compound , average hardness is HV800, metallurgy combines with the base,improves percent 149 compare with H13 hot work die steel .