The decoherence dynamics of a two-level atom coupled into a vacuum electromagnetic field are studied. The analytic solution of the Schrodinger equation has been given

and then the reduced density matrix for the motion of the atomic center of mass has been derived. It is shown that spontaneous emission induces the entanglement between the atomic spatial motion and the atomic electronic states

which induces the spatial quantum decoherence of the atom. The decoherence factor is given

and its properties

such as dynamical evolution with time and evolution for different atomic positions

are demonstrated by some figures.