In this research, a general formula for the Casimir force acting on a dissipative dielectric slab located next to a perfect conducting plate has obtained. The dielectric function of the slab is assumed to be an arbitrary complex function of frequency satisfying Kramers-Kronig relations. A classical expression for the radiation pressure of the vacuum fields on the slab is rendered by using the Maxwell stress tensor. With the transition to the quantum domain and using the fluctuation dissipation theorem and Kubo? s formula, the resulting expression is written in term of the imaginary part of the vector potential Green function ? s components of the system. Finally, by computing the Green function, the Casimir force on the slab is obtained. This formalism enables us to calculate the Casimir force without resorting to the explicit form of the field operators.