The entanglement of two two-level atoms in a coherent field of time-varying frequency have been investigated. The properties of the entanglement of two atoms for two typical——the frequency of field varying with time in the forms of sine function and rectangular pulses——have been considered. The numerical calculation show: for the frequency changing with time in sinusoidal form, the frequency and the mean photon number of field affect the entanglement of two atoms and in the condition of a small number of photons, small-amplitude fast oscillations occur during half period. For the frequency changing with time in rectangular form, The sudden jumping of the field with rectangular pulsed frequency-modulation can change the coherence of the two atoms, and induce the new entanglement phenomenon of the two atoms. The entanglement of two atoms is small-amplitude fast oscillations during the pulse. The changes of optical field frequency can change the coherence of atoms, and then change entanglement between atoms. The paper prove to be constructive in entanglement preparation and control in quantum information technique.