We investigate the entanglement time evolution of two entangled two-level atoms that interact resonantly with a single-mode field in the Fock state. The results show that the two-atom entanglement state appears with periodicity. The influence of the two-atom initial state, the dipole-dipole coupling intensity between two atoms, and the field in the Fock state on the entanglement degree of two atoms are revealed. Meanwhile the two-atom quantum state will forever stay in the maximum entangled state when the initial state is proper.