Properties of catalysts, such as activity and selectivity, often depend on their formation and reduction processes. In this paper, formation and reduction processes of ultrafine particles of Fe/Mn oxide catalyst are simulated by the Monte Carlo method. From the simulations, the reduction rate as a function of reaction time exhibits a typical S-curve observed in many experiments. By further analysis, it can be found that the reduction reaction begins in the interior of the oxide particle, thus the structure of the particle can obviously change during the reduction process and the enrichment of Mn component at the surface is due to the combination energy of Mn which is less than that of Fe.