Abstract: Kapton is frequently used as the base of thermal control materials on the surface of spacecrafts. Atomic oxygen undercutting at defect sites in the coating is a normal threat to these protective coated Kapton in LEO space flight. In this paper, Monte Carlo method is used to determine the dependence of atomic oxygen undercutting erosion upon defect size. Results indicate that the configuration of atomic oxygen undercut cavity is different with different defect sizes. The wider defect has a more flat undercut cavity, which is wider at the bottom than at the top. The width of the atomic oxygen undercut cavity varies with the thickness of the protective coatings, and the depth of undercut cavity also sees a small change. In summary, the numerical simulation results can provide a useful guide to develop new materials and protective coatings for aerospace applications.