We investigate non-Hermitian Hamiltonian which governs system includes two-level atoms and electromagnetical field. Using the notion of negativity, we study the degree of entanglement of a two- level atom interacting with a quantized electromagnetical field, described by the non-Hermitian Hamiltonian (Saaidi in Phys. Scr. 77:0065002, 2008). With the help of numerical calculation for the case that the system state is pure, we show that the measurement of negativity of this system is nonzero and has a different functional with respect to negativity of the Jaynes-Cumming model (JCM).