Silicon surfaces after Ar ion‐assisted etching in Cl2gas atmosphere were studied for various Ar ion current densities and Cl2gas pressures. The number of incident Ar ions was varied in 6×1014–3×1016ions/cm2/s while the number of incident Cl2molecules was varied in 0–3×1017molecules/cm2/s. Ar ion acceleration energy was fixed at 1 keV. High etching rate, about 5000 Å/min, was observed with a high incident rate of both Ar ions and Cl2molecules. On the other hand, high etch yield was observed at low incident rate of Ar ions and high incident rate of Cl2molecules. The amount of surface defects and surface chemical adsorption states, which vary with different etching conditions, were studied by etch pit observation after thermal oxidation followed by Secco‐etching, x‐ray photoelectron spectroscopy (XPS), and reflection high energy electron diffraction (RHEED). Silicon surfaces, etched under high etch yield etching conditions, showed relatively high Si2O3existence by XPS measurement, and high density etch pits. It is considered to be related with high density surface defects and roughness. Etch pits did not appear after conventional wet cleaning process. It is caused by a little surface oxidation and relatively less Si2O3existence than other kinds of oxides on the surface.