ZnO nanopowders were synthesized by thermal physical method, which present two types of morphology, such as nanoparticles and nanowhiskers. The produced tetrapod nanowhiskers usually show four needle-like feet that originate from the center of the polyhedron. The nanowhiskers display a fantastic optical property of absorbing light, which is totally different from the nanoparticles. The nanowhiskers can enhance the absorptive ability of coating for CO 2 laser beam. The results show that the hardness and depth on the laser-hardened surface of steel 5Cr5MoSiV are improved dramatically. Compared with laser-hardening by using absorbent without the nanowhiskers, the hardening only demanded an 800 W output for CO 2 laser, saving the power as 30%.