  For both cases with and without interactions, bipartite entanglement of
two-fermions from a Fermi gas in a trap is investigated. We show how the
entanglement depends on the locations of the two fermions and the total number
of particles. Fermions at the verge of trap have longer entanglement distance
(beyond it, the entanglement disappears) than those in the center. We derive a
lower limitation to the average overlapping for two entangled fermions in the
BCS ground state, it is shown to be $\sqrt{Q/2M}$, a function of Cooper pair
number $Q$ and total number of occupied energy levels $M$.
