  A cluster mean-field method is introduced and the applications to the Ising
and Heisenberg models are demonstrated. We divide the lattice sites into
clusters whose size and shape are selected so that the equivalence of all sites
in a cluster is preserved. Since the strength of interactions of a cluster with
its surrounding clusters is strongly dependent on the spin configuration of the
central cluster itself, we include this contribution in the effective fields
acting on the spins. The effects of "correlations" between clusters can be
taken into account beyond the standard mean-field level, and as a result our
cluster-based method gives qualitatively (and even quantitatively) correct
results for the both Ising and Heisenberg models. Especially, for the Ising
model on the honeycomb and square lattices, the calculated results of the
critical temperature are very close (overestimated by only less than 5 %) to
the exact values.
