Scanning capacitance microscopy (SCM) is employed to examine iron-contaminated p-type Si samples. For slightly contaminated samples, a dc voltage of -0.8 V, applied between the sample and the conductive tip, induces positive trapped charges. Owing to the existence of these charges, the region containing trapped charges exhibits an obviously low dC/dV signal. According to contact-mode atomic force microscopy results, the surface morphology has little effect on the SCM signal. The experimental results indicate that SCM is capable of detecting the distribution of oxidation-related defects which cannot otherwise be easily observed by atomic force microscopy and transmission electron microscopy.