In this thesis we have studied the static Casimir effect between two perfect conducting slabs hn Fabry- Perot cavity at thermal state. An expression for the radiation pressure of the vacuum fields on body immersed in it is introduced by using the fluctuation dissipation theorem and kubo's formula in a gauge in which the scalar potential vanishes, the radiation pressure is written in terms of the imaginary part of the vector potential Green function of the system and temperature.