  In this paper, we find the invariant for $n$-qubits and propose the residual
entanglement for $n$-qubits by means of the invariant. Thus, we establish a
relation between SLOCC entanglement and the residual entanglement. The
invariant and the residual entanglement can be used for SLOCC entanglement
classification for $n$-qubits.
