  We obtain a simplified and obvious expression of "concurrence" in Wootters'
measure of entanglement of a pair of qubits having no more than two non-zero
eigenvalues in terms of concurrences of eigenstates and their simple
combinations. It not only simplifies the calculation of Wootters' measure of
entanglement, but also reveals some its general and important features. Our
conclusions are helpful to understand and use quantum entanglement further.
