We report an experimental investigation of the pits formed at oxidation. The pits were formed by a long time oxidation in a dry or wet atmosphere at high temperature (~ 1200 oC). Although they were observed in (0001) face, they were not in (000-1) face. Comparing the points of oxidation pits to those of molten KOH etching pits on the same area of the substrate, we show that the oxidation pits are formed by oxidizing defects. In addition, the influence of the oxidation pits to the reliability of thermal oxide was researched.