Jingzhou Teachers Coll, Dept Phys, Jingsha 434104, Peoples R China
; Chinese Acad Sci, Inst High Energy Phys, Beijing 100039, Peoples R China
; Huazhong Normal Univ, Inst Particle Phys, Wuhan 430070, Peoples R China
A spectral representation for two-point Green function for arbitrary composite field operators is derived in Thermo Field Dynamics (;TFD). The application to two-to-two scattering at finite temperature shows that the corresponding cross section is of the form sigma=sigma(d)-sigma(i) for two bosons or two fermions, which is similar to the decay rate Gamma=Gamma(d)-Gamma(i) for single boson. The cross section sigma(d) for the scattering process and sigma(i) for the corresponding inverse process determine the rate of approaching to equilibrium of the particle distribution function.