A direct method for lateral patterning of silicon on insulator (SOI) films with sub-mum resolution is presented. This method is based on rapid thermal oxidation induced by a focused laser beam. By focusing the cw light of an argon ion laser at a wavelength of 458 nm (514 nm) we achieve a diffraction limited laser spot of 315 nm (350 nm). The laser spot is scanned over the surface of a 15-100 nm thick silicon film in ambient air. Above a critical laser power rapid local oxidation of the entire silicon him at the exposed sample spot is observed. Below this threshold power even within longer time scales no changes of the sample surface are detected. AFM measurements of laser written oxide lines show line widths down to 200 nm. Both this high spatial resolution and the dynamics of the oxidation process near the threshold power are attributed to nonlinear effects of the absorption and heat conduction in the sample. Model calculations show that the temperature profile can be even narrower than the laser spot diameter The oxidation is related to an unstable temperature distribution in the exposed sample, caused by a self amplifying heating.