With a computer controlled excimer laser marking system, we studied marking processes of stainless steel. On the conditions of different laser fluences, the laser irradiated surface of stainless steel appears different. The marking threshold (the laser fluence corresponding to the melting point of iron) can be concluded by calculating the laser induced surface temperature. As the laser fluence exceeds the threshold a mirror surface appears, otherwise, an oxide layer will be formed. The calculation shows that the rates of surface temperature variation (heating rates or cooling rates) of 1010 ℃/s can be obtained, which neans a noncrystalline layer should be formed on steel.