The giant impact (GI) hypothesis is one of the most important hypotheses both in planetary science and in geoscience, since it is related to the origin of the Moon and also the initial condition of the Earth. A number of numerical simulations have been done using the smoothed particle hydrodynamics (SPH) method. However, GI hypothesis is currently in a crisis. The “canonical” GI scenario failed to explain the identical isotope ratio between the Earth and the Moon. On the other hand, little has been known about the reliability of the result of GI simulations. In this paper, we discuss the effect of the resolution on the results of the GI simulations by varying the number of particles from 3 × 103 to 108. We found that the results does not converge, but show oscillatory behaviour. We discuss the origin of this oscillatory behaviour.