Self-powered wearable electronic devices are expected to be one of the mainstream technologies for future portable electronic systems. As an energy harvester to power wearable electronic devices, a flexible thermoelectric generator (f-TEG) can utilize human body heat as the energy source, creating an ideal solution. The fTEG can make conformal contact to the human skin and fully utilize body heat with minimal energy loss, while also being comfortable to wear. However, to maximize the power generated by an f-TEG attached to the human body, a careful thermal and structural design analysis of the f-TEG must be carried out. Here, we fabricated flexible thermoelectric generators (f-TEGs) with different device parameters and evaluated their power generating performance using an artificial arm, which was carefully designed to mimic a real human arm. We demonstrated the impact of the f-TEG device parameters on the power generation performance under various circumstances. The experimental results were compared with the theoretical model, and guidelines for an optimum device design in terms of maximizing the generated power density are also presented. Finally, we show that the optimum device structure varies when the efficiency of the power management IC (PMIC) is included in the analysis of the power generation system, which is practically important.