Pitting corrosion has been one of the main corrosion types of immersed ship hulls, which can bring heavy damage and even accidents to in-service ships, particularly to aged ships. The aim of the present paper was to investigate the effects of pits on the ultimate compressive strength of mild steel plates under uniaxial compression. A series of nonlinear FEM analyses on plates with partial depth corrosion pits were carried out, changing the size, intensity and location of pits and the slenderness of plates. It was shown that the eccentricity induced by single side distributed pits had considerable degrading effects on the ultimate compressive capacity of plates. Although the degree of pit corrosion (DOP) did reflect the effect of pits to a large extent, it was not enough to rely on DOP exclusively to represent the extent of damage caused by pits, as “volume effect” should be considered.