  In this paper, we present a general formula for obtaining the reduced density
opeator for any biparticle pure entangled state. Using this formula, we derive,
in a compact form, the explicit formula of the entanglement for any bipartical
pure entangled Gaussian state. In the case of Gaussian states, the criteria of
separabelity can be naturely obtained by the formula. For non-Gaussian states,
we also show the usefulness of the method presented in this paper.
