We investigate the robustness of entanglement for a multiqubit system under dephasing and bit flip channels.We exhibit the difference between the entanglement evolution of the two forms of special states,which are locally unitarily equivalent to each other and therefore possess precisely the same entanglement properties,and demonstrate that the difference increases with the number of qubits n.Moreover,those two forms of states are either the most robust genuine entangled states or the most fragile ones,which confirm that local unitary (LU) operations can greatly enhance the entanglement robustness of n-qubit states.