Abstract
We study the influence of temperature on the nonlinear effective action of QED, in the presence of constant external fields, by means of the Fock-Schwinger proper time method. The analysis is done according to thermo field dynamics. This is an extension of the work done by Dittrich and we comment on the case of a constant electric field. In this way we find an effective thermal Euler-Heisenberg Lagrangian, identifying the effective thermal coupling constant and an effective thermal mass for the photon.