  The giant impact (GI) is one of the most important hypotheses both in
planetary science and 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 $3
\times10^3$ to $10^8$. We found that the results does not converge, but shows
oscillatory behaviour. We discuss the origin of this oscillatory behaviour.
