We have studied the effects of disorder on surface stress during oxidation. The surface stress change during ion bombardment and the following plasma oxidation on Si(100) was measured by means of an optical microcantilever technique. We have found compressive stress on Si surface due to disorder induced by ion bombardment and determined it quantitatively in terms of the number of defects. This disorder-induced compressive stress was completely relaxed by the plasma oxidation processes. The initial evolution of the surface stress during oxidation on bombarded surfaces is quite different from that on unbombarded Si(100) surfaces.