Precipitation behavior of magnetite particles in the thermal grown oxide scale during isothermal cooling of microalloyed low carbon steel was studied using scanning electron microscopy (SEM) and thin film X-ray. The oxide scale was generated from Gleeble 3500 Thermal-Mechanical Simulator connected with a humid air generator, to simulate 550 and 450°C isothermal treatments. Several types of magnetite precipitates were observed during different cooling processes with respect to the possible mechanisms of precipitation have been discussed. It is found that magnetite particles is as a result of pro-eutectoid precipitation from oxygen-rich wustite, and also as a product of the partial decomposition of wustite during the cooling process due to change of oxygen concentration and migration of iron ions. Furthermore, microalloyed elements in steel reduce the stability of wustite thereby facilitate the precipitation process, whose products of multi-phase oxide finally determine the adhesive strength of oxide scale and steel substrate. © (2012) Trans Tech Publications, Switzerland.