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