In this article, we report the fabrication of Si nanopillars by surface reaction with nitric oxide (NO) at high temperature. NO was leaked onto clean Si surfaces, at a temperature of similar to 850 degrees C, in ultrahigh vacuum chamber and examined in situ by x-ray and ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, low energy electron diffraction, and ex situ by atomic force microscopy. NO molecules dissociate on the surface and nitrogen atoms thus produced form nitride islands. These islands act as protective masks for the etching of Si by the oxygen atoms, through the desorption of SiO species. Occurrences of these two simultaneous processes results in the formation of nanometer sized Si pillars (typical basewidth 100-150 nm and height 5-15 nm) capped by silicon nitride. These pillar structures are separated by clean Si areas as shown by the clear presence of two domains. We also show that the height of these Si pillars can be increased by ex situ chemical etching.