In this paper, we investigate twoatom entanglement in time evolution and the thermal entanglement in a generalized JaynesCummings model by means of concurrence. The result shows that the twoatom entangled state appears with periodicity, which decreases with increasing dipoledipole coupling between the two atoms. In a finite temperature, the concurrence decreases with increasing temperature, and the thermal entanglement vanishes in this JaynesCummings model when the system temperature reaches the critical temperature, which is proportional to the dipoledipole coupling strength of the two atoms. For typical experimental data, the critical temperature is on the order of 10-5K. In addition, there exists the quantum phase transition in this JaynesCummings model