The CO oxidation reaction on DLA fractal surface has been studied in detail by Monte Carlo method. It was found that (1) when only adsorption and surface reaction were considered the O atoms and CO molecules on surface would self-organize after a reation period, which greatly decreased the touching chance among different reactant molecules and consequently decreased the reaction rate. Furthermore, O atoms tended to be adsorbed at the center or the inner area of DLA surface while CO molecules could exist only at the exterior; (2) the clusters of O atoms could be cut off by vacant active sites after the introduction of CO diffusion on surface, which would further give rise to accelerateion of the reacti0n rate; (3) when the reversible adsorption of CO was introduced O atoms and CO molecules could adsorb evenly on the whole DLA surface and then the reaction would be speeded up greatly, which suggested that for a good catalyst the adsorbing strength should be moderate.