Spherical drilling and staining was studied and found to be one of the simplest methods for measuring the thickness of Si epitaxial layers, especially when their thickness was less than one micron. The minimum thickness which could be measured by this method was about 1000A with an error of less than 80A (8%). The results obtained by this method were compared with those by other methods i. e., beveling and staining, weight difference, and infrared reflection. This method was also applied successfully to the thickness measurement of dielectric or thin metal layers on Si substrate,