The burning mechanism which occurs in oxygen assisted laser cutting of mild steel and the resulting periodic cut edge striations are under consideration in this study. The average wavelength of the periodic striations is shown to be independent of the processing speed, but strongly correlates with the beam radius at the work piece surface. A theoretical approach has been developed to describe the transient oxidation process. Initially, the heated iron at the cutting front oxidizes rapidly and the oxidation front can retreat from the laser beam. The growing oxide layer acts as a barrier for diffusion, lowering the reaction rate which in turn reduces the exothermal energy gain. Consequently, after few 10 µsec the oxide layer of some microns thickness terminates the retreating motion. Typically after 10 msec the reaction is terminated until the next, periodical ignition by the laser beam occurs. In order to monitor the periodic process, the thermal emissions from the cutting front are detected by employing a photodiode and analyzing the temporal signal. An analysis algorithm suitable for on-line detection of both the roughness of the striations and the appearance of defects is presented.