  We reexamine the screening mechanism in $f(R)$ gravity using N-body
simulations. By explicitly examining the relation between the extra scalar
field $\delta f_R$ and the gravitational potential $\phi$ in the perturbed
Universe, we find that the relation between these two fields plays an important
role in understanding the screening mechanism. We show that the screening
mechanism in $f(R)$ gravity depends mainly on the depth of the potential well,
and find a useful condition for identifying unscreened halos in simulations. We
also discuss the potential application of our results to real galaxy surveys.
