The thermal entanglement produced by resonant Jaynes-Cummings model with and without the effect of virtual-photon field is investigated.It is shown that there exists a critical temperature,above which thermal entanglement of atom and field mode vanishes.In the absence of virtual-photon field,the critical temperature increases with the increasing of the coupling constant between atom and cavity field.Thermal entangelement can be generated for sufficiently strong coupling under room temperature.In the presence of virtual-photon field,it is also shown that the critical temperature is determined by the processes of real and virtual photon transition.When the process of virtual photon transition is equal to that of real photon transition,no thermal entanglement takes place even if the setup is cooled to below the critical temperature.