We study the entanglement evolution of two three-level atoms in a fermionic environment. Our results show that the entanglement evolution depends not only on the strength of the interaction between the system and environment， but also on the structure of states. For the cases of concern， we find that the stronger the interaction between the system and environment is， the faster the entanglement of the system decreases. The entanglement of pure quantum states vanishes completely when the time goes to infinity; while for the mixed state，the entanglement will be completely destroyed by decoherence in finite time. The decoherence-free subspace SDF has been identified by using linear entropy to measure decoherence. A density matrix that only can be expanded by the elements of SDF does not perceive the presence of the environment and the entanglement and its linear entropy remain unchangeable. Our analysis will shed some light on the effect of a fermionic environment on the entanglement of bosonic systems.