A theoretical analysis was developed to present the influence of the temperature photodarkening effect on the performance of high power ytterbium doped double-clad optic fiber lasers. To support the theoretical analysis, three fundamental equations were used to describe the heat generation in the fiber core, the intrinsic light absorption, the quantum defect, and the influence of the photodarkening effect in the laser performance. With these equations in hand it was possible to investigate the behavior of the total heat flux in the fiber and the temperature distribution, both in the fiber???s core in the steady state regime. The laser model presented in this study was developed to demonstrate the fundamental importance of the photodarkening effect in the heating generation of silica fiber lasers in a tandem pumping configuration.