This paper presents the bond graph approach as a graphical representation for the dynamic modeling of an exoskeleton actuator system, comprising electrical brushless DC motor, harmonic drive gearbox, torque sensor and human limb. The main purpose of this paper is to show some advantages of the bond graph method in order to model exoskeleton actuator systems for the better understanding of the system dynamics. A block diagram dynamic model of the system is briefly discussed and also for finding the friction model parameters and the friction torque, an experimental test has been carried out. Both the block diagram model and the bond graph model are simulated through MATLAB software and the bond graph simulation software 20-sim. Finally, simulation results in terms of two bond graph effort and flow variables are presented and discussed. The proposed work is generally applicable to extract dynamic model of all kind of wearable robot actuators.