A technique for fabricating high-aspect-ratio Ni structures has been developed using wet Si processing. Si(100) was initially etched in HF to form high-aspect-ratio (exceeding 200) micrometer-sized pores. By introducing the as-etched Si into a defined Ni2+ solution, Ni deposits grew rapidly and replaced the Si on the sidewalls. After long immersion, the Si frame was completely converted to create a Ni structure with high-aspect-ratio pores. Close-packed submicrometer Si pillars can be formed by oxidizing and etching of the macropore sidewalls. Treated in the same Ni solution, high-aspect-ratio Ni pillars were correspondingly generated by replicating the pillar-type Si structure.