  In this paper, by using the notion of negativity, we study the degree of
entanglement of a two-level atom interacting with a quantized radiation field,
described by the Jaynes-Cummings model (JCM). We suppose that initially the
field is in a pure state and the atom is in a general mixed state. In this case
the negativity fully captures the entanglement of the JCM. We investigate the
case for that the initial state of the field is a coherent state. The
influences of the detuning on the degree of entanglement is also examined.
