Strength and deformability of steel plates for marine use are studied from the viewpoint of geometry of corrosion pits. In the present study, the effect of actual pit shapes has been investigated by the non-linear, large deformation and three-dimensional finite element analyses for simulated corrosion surfaces generated by a probabilistic model of corrosion process. The tensile strength of corroded plates with semi-ellipsoidal pits is found to be estimated by the empirical formula obtained from that with conical pits,where the estimation is based on the minimum cross sectional area of the plate. The deformability of the corroded plates subjected to uniaxial tension could be estimated by the surface roughness represented by the difference of averaged plate thickness and that at the section of minimum cross sectional area. This study also shows that the reduction of the bending strength due to semi-ellipsoidal pitting corrosion is higher than that of compressive/shear/tensile strength. An empirical formula for the reduction of bending strength due to semi-ellipsoidal pits is proposed which can be a useful tool for condition assessment of aged marine structures.