A dynamic Monte Carlo simulation is proposed to explore the effect of inert surface impurities on the oscillatory behavior and pattern formation in the CO oxidation by oxygen on Pt(100) surface at ultra high vacuum conditions (UHV) and relatively high temperature conditions. In this study it was found that the oscillation frequencies and the pattern reaction fronts, in this case cell type, decrease when the amount of impurities increases. This occurs mainly because the adsorption of O₂ from gas phase decreases and the diffusion process of adsorbed CO is delayed. Thus oscillations and spatial patterns disappear when the amount of impurities on the substrate is greater than 15 %.