The effect of adding HCl during annealing in argon with reduced oxygen to suppress the generation of oxidation-induced stacking faults (OSFs), which adversely affect the yield and reliability of thermally grown oxide, was investigated for p-type Si wafer. Adding HCl affected OSF generation in two ways depending on the oxygen partial pressures above and below the critical value at 6 x 10(-3) atm. When the oxygen partial pressure was lower than the critical value, adding HCl did not suppress this generation any more than the degree brought by annealing without HCl, when an OSF density of about 10(3) cm(-2) was reduced to 20 cm(-2) or less. At an oxygen partial pressure greater than the critical value, adding HCl suppressed the generation of OSFs almost completely. The mechanism responsible for the two ways to suppress OSF generation by adding HCl is discussed, including the oxidation mechanism when oxygen partial pressures are reduced. This difference is attributed to the difference in the transport mechanisms of oxidation-related species at reduced oxygen partial pressures.