Thermoregulatory clothing that can assist the human body in maintaining thermal comfort will not only improve quality of life and chance of survival in wide range of environments, but also result in huge energy saving used for regulating the indoor thermal environment. Development of effective thermoregulatory clothing however represents an enormous challenge. Here, for the first time we show a lightweight thermoelectric air conditioning undergarment system with a branching tubing network capable of supplying sufficient heating and cooling power to expand the ambient temperature by 2.2 ºC (4ºF) on both hot and cold sides of the neutral band without comprising thermal comfort, which can potentially bring about 15% saving in HVAC energy consumption. It was demonstrated that the performance of the air cooling/heating system can be optimized by using the optimum operation conditions of energy input and the optimal tubing diameters at different levels. In addition, the heating/cooling effect of the personal air thermoregulatory system is also affected by the interactions between human and clothing, such as body postures and clothing design. It was found that sedentary posture can lead to higher heating and cooling power than standing posture. Leaning position for sedentary posture can further improve the performance in cooing condition. These improvements are mainly attributed to the change in heat transfer efficiency at chest and back areas, where air flow is directed to. It was also found, the position of wind outlets for a personal air thermoregulatory clothing has influence on the cooling/heating performance.