Silicon nanorods 20 nm in diameter are fabricated by reactive ion etch (RIE) to study anisotropy and damage profile in decananometer scale. RIE of gas mixture of SF6/O2 and SF6/CHF3 is tuned to achieve high anisotropy. The gas specie of SF6/O2 can reach 90% anisotropy, 84° taper angle, and 10:1 selectivity when SiO2 is used as the etching mask. The gas species of SF6/CHF3 can reach 95% anisotropy, 87° taper angle, and 10:1 selectivity with Cr as the mask. The fabrication technique of nanorods uses a monolayer of silicon dioxide nanoparticle as the etching mask. The nanorods uniformly cover up the entire 2 in. wafers with high density of 2×1011 cm-2. Surface damageafter the etching process of nanostructures is monitored using the microwave-reflectance photoconductance decay with KOH removal-and-probe technique. Highly damaged silicon is found within a depth of 30 nm and the lightly damaged part extends more than 100 nm.