Abstract: Simulation tests for oil-based mud contamination of the cores sampled from an oilfield of South China are carried out in laboratory, which proved that oil-based mud not only changed the wettability of cores into a strong lipophilicity, but also caused the rock pores being blocked by the solid grains in oil-based mud, as a result,there is a great reduction in effective and absolute permeabilities of oil phase. Once cores are damaged by oil-based mud, it is difficult to restore the permeability of cores to its original level by applying the conventional technique of water injecting and washing.