A controlled experiment was performed to determine whether residual stresses in silicon oxide films affect the apparent modulus and hardness of the films measured using microindentation. A set of wafers, each with a SiO2 film of different residual stress, were produced by PECVD deposition using different RF power settings. Residual stresses, determined by curvature measurement, were shown to become more compressive with the increase of the RF power. Correlation between indentation modulus/hardness and residual stress was established because the modulus was also observed to be trending with the RF power. To determine whether the residual stress change is the cause for the change of indentation modulus/hardness, one of the wafers was cut to pieces and bonded to steel substrates of different thickness at elevated temperatures. Because of the thermal mismatch between silicon and steel, the silicon pieces were stressed by the steel substrates and therefore provided additional stresses to the SiO2 film on the top surface. The applied stresses were determined using Raman piezospectroscopy. Micro-indentation tests performed on these samples showed no appreciable change of modulus/hardness due to additional stresses. It was concluded that the material change, such as density, was behind the correlation between indentation modulus and residual stress.