A new parameter was proposed to evaluate the fatigue life of toothed belts. The parameter is the frictional work spent on the belt tooth surface for driving and driven pulleys. It can be estimated only with the 2D finite element model of the belts previously developed by the authors. As well as the frictional work, an alternative parameter, maximum tooth load (widely used in the literature) was also used to evaluate the fatigue life of toothed belts. In order to prove the effectiveness of the present parameter, fatigue tests were conducted using S8M belts at a constant power. The test results show that the maximum tooth load can explain the fatigue degradation of the toothed belts to some extent while the proposed parameter, the frictional work can evaluate the fatigue life of the belt due to wear of the belt facing fabric more appropriately than the maximum tooth load.