Purpose : The conception of a dynamic model for a robot, which can simulate its behavior in the presence of gear transmission defects. Method : Based on the Denavit-Hartenberg Modifed (DHM) method for geometric modeling of robots and on the Euler-Lagrange approach, which is founded on an energetic calculation, a new method has been proposed for the dynamic modeling of robots. This new method is based on a new way of modeling the articular fexibility of the robots, in particular that due to the gear transmission systems. This methodology also gives the possibility to integrate the gear defects in the dynamic model of a robot. Results : With the new approach a dynamic model for a three-axis robot has been designed. The dynamic response of this model showed the efect of meshing between the teeth of the gearbox wheels on the behavior of the robot. On the other hand, the behavior of the robot was simulated in the case where there is a gearing defect (tooth cracking defect). Conclusion : Finally, this new approach provides a fexible dynamic model that can be used to analyze the behavior of the robot in the healthy case and after the introduction of a gear fault. In other words, this numerical model can be used to diagnose and predict gear defects that may occur in the robots articulations.