  We construct an analytical model of the mass distribution function of
subhalos in a galaxy cluster in the context of the cold dark matter (CDM)
theory. Our model takes account of two important effects; the high density of
the precluster region and the spatial correlation of initial density
fluctuations. For subhalos with small masses, the mass distribution function
that our model predicts is intermediate between the Press-Schechter (PS) mass
distribution function and a conditional mass distribution function as an
extension of the PS formalism. We compare the results of our model with those
of numerical simulations. We find that our model predictions are consistent
with the mass and velocity distribution functions of subhalos in a cluster
obtained by numerical simulations. We estimate the probability of finding large
X-ray subhalos that often have ``cold fronts''. The observed large X-ray
subhalos and cold fronts may not always be the result of cluster mergers, but
instead may be internal structures in clusters.
