Flexible thermoelectric generator became an attractive technology for its wide use especially for curved surfaces applications. This study proposes design of a flexible thermoelectric generator, which is part of a sensor and supplies required electrical power for human body application. The thermoelectric generator module has ink based thermoelements which are made of nano-carbon bismuth telluride materials. Flexible fins conduct the body heat to the thermoelectric uni-couples, extended fins exchange the heat from the cold side of the thermoelectric generator to the ambient. A fully developed one-dimensional steady-state numerical model including temperature-dependent thermoelectric properties of different materials is built to reveal basic characteristics and optimal design criteria of the thermoelectric generator. Results show that ambient temperature presents significant influence on thermoelectric generator performance, but thermal resistance from blood to skin surface and contact thermal resistance at skin surface has negligible influence. A very low air velocity, e.g. 0.01 m/s is enough to ensure a considerable temperature difference through the thermoelectric element. Increasing thermoelectric elements thickness and thermoelectric module row number in a proper range can significantly enhance thermoelectric generator performance. The maximum output power can reach 0.2 W/cm(2), which indicates the proposed design is promising for supplying human body sensors. In addition, the basic optimal design criteria of the flexible thermoelectric generator and its relative merits are discussed and presented.