Based on the entanglement degree of two subsystem which can be measured by the negative eigenvalues of the partial transposition matrix,time evolution of entanglement degree of two atoms in the Tavis-Cummings model,which is interacted with a field in the Fock state,is investigated by numerical methods. When these two atoms are initially prepared in mixed states,the results show that the field in the Fock state,the initial state of two-atom and the coupling constant between the field and the two-atoms have a dramatic influence on the time evolution character of entanglement degree of two atoms.