Grease life in ball bearings is reduced by increasing load. In this paper, we demonstrate that this effect is, at least in part, driven by an increase in temperature at the bearing raceway. This temperature rise results from heat generated by sliding friction between the balls and the raceway, which in turn accelerates the oxidative degradation of the grease. The temperature is estimated using the flash temperature concept. By substituting the contact temperature in the ’Arrhenius equation’ for the expected grease life, as used in the bearing community, a relation between load and grease life is established. These calculations match well with the measured grease life dependence on bearing load. The result is a relatively simple predictive model for grease life under varying loads.