An experimental investigation was made of the formation of thermal pits and hillocks produced on (111) and (100) surfaces of silicon by heating in the temperature range 800°C-1, 300°C in a high vacuum at 3×10^-7 Torr and 3×10^-8 Torr. It was found that the formation of the thermal pits are caused by preferential thermal etching of surfaces contaminated with initial adsorbates and adsorption of residual gases. Contamination by initial adsorbates was found to form the oriented thermal pits. Residual gases at 3×10^-7 Torr were found to form hillocks due to reaction with Si. The presence of β-SiC was always observed on RHEED patterns obtained from surfaces having the thermal pits and hillocks. The effect of thin oxide films on silicon were also investigated. No thermal pits were observed on the surfaces heated with oxide films, even at 3×10^-7 Torr.