In crack propagation analyses of ship side outer platings subjected to collision load, effect of prestrain on crack propagation behavior should be clarified because not a few plastic deformation is usually generated on outer platings before crack initiation. And crack propagation simulation methods using a coarse mesh FEM model would be required in ship collision analysis owing to a huge numerical simulation model. In this paper, pre-strain effect on crack propagation behavior was investigated with tensile tests of center notched plates whose pre-strains were 0.0, 0.15 and 0.3. In the experiment crack tip opening angles measured from grid line deformation drawn on the specimens were obtained for each pre-strain level and were verified to be a useful parameter for crack propagation simulation by performing FEM analyses of the experiment. Then a practical FEM simulation method for crack propagation, which includes the finite element size effect, was proposed.