The fidelities of quantum states in the system of two-mode entangled coherent field interacting with atoms in the Bell states in Kerr thin medium are investigated by means of full quantum theory. The results indicate that for ideal cavity, the fidelity of the system always equals to that for the coherent maintenance state at original time when average photon number of cavity fields is very small. When average photon number of cavity fields increases, the fidelity of the system is humdrum attenuation. The fidelity of the system oscillates periodically from 0 to 1 for other Bell states at original time when average photon number of cavity fields is very small. When average photon number of cavity fields increases, oscillatory frequency of the fidelity increases and amplitude decreases. For fine cavity, the fidelity is humdrum exponent attenuation for the coherent maintenance state at original time. The fidelity is oscillatory exponent attenuation for other Bell states at original time, and when average photon number of cavity fields increases, oscillatory frequency of the fidelity increases.