We study the entanglement of thermal and ground states in Heisernberg XX qubit rings with a magnetic field. A general result is found that for even-number rings pairwise entanglement between two nearest-neighbor qubits is independent on both the sign of exchange interaction constants and the sign of magnetic fields. As an example we study the entanglement in the four-qubit model and find that the ground state for this model without magnetic fields is shown to be a genuinely four-body maximally entangled state.