Abstract: In order to determine the collapse strength of casing with crescent-shaped inner wall wear, finite element analyses are conducted on the basis of plane strain, plane stress and spatial problem, respectively. It is shown that the calculated collapse strength is much less than the one looked up from the manual in reference to minimum wall thickness. In addition, the strength decreases with the increase of wear depth or/and the decrease of casing diameter. It is found that the plane treatment of the problem is acceptable if the axial dimension of the wear is large enough.