Tooth stiffness of toothed belts having trapezoidal teeth was numerically analyzed using the nonlinear finite element method. Here, friction between the belt tooth and the pulley, and large deformation of the tooth were considered. Numerical results reveal that the observed tooth stiffness is strongly affected by the belt tension and number of teeth of the pulley, although the relationship between a tooth load and tooth deformation can be considered linear for a given condition. It is also found that the observed tooth stiffness increases with an increase of average belt tension and reaches a certain value. The friction acting on a side surface of the belt tooth has little effect on the observed tooth stiffness.