Wind sensors are widely used in many critical applications, such as navigation, weather forecast, aerospace and wind energy assessment. The thermal wind sensor can measure wind speed and direction at the same time, by measuring the temperature gradient on the sensor’s surface which is induced by the wind flow. Compared to traditional mechanical wind sensors, there are two main advantages for thermal wind sensors: Firstly, thermal sensors can measure the wind flow without any moving parts, thus no maintenance is required. Secondly, the sensing elements of the thermal wind sensor (such as heaters, thermopiles, etc) as well as its interface circuitry can be realized in a single CMOS chip. Therefore, the robustness of the sensor system can be enhanced. Despite these advantages, the existing CMOS thermal wind sensor’s relatively high power consumption strictly limits its applications. The scope of this thesis is the design and realization of a low-power CMOS thermal wind sensor with the competitive sensing accuracy as its mechanical counterparts. In order to achieve this, the constant power mode instead of traditional constant temperature difference mode is applied to the thermal wind sensor to reduce the power consumption. The advantage of the constant power mode is that the power consumption of the sensor can be controlled by an external digital signal, instead of being limited by the sensing inaccuracy of the temperature sensors. Based on the existing sensing elements, only by adding an extra comparator, the air-flow induced temperature gradient ?T can then be directly converted into digital bit-stream by a thermal ?? modulator. Because the noise-shaping provided by the thermal loop filter of the first-order thermal ?? modulator is not sufficient, for given sensing resolution, the heater’s power consumption of the thermal wind sensor is limited by the quantization noise. By cascading the thermal filter with an electrical integrator, the resulting second-order thermal ?? loop can provide more noise-shaping than the first-order system, to further reduce the quantization noise, thus required heating power consumption. To achieve reduced error due to the self-heating of the circuit, the chopping amplifier instead of traditional auto-zeroing amplifier, is used to interface the thermopile output signal. Two prototypes of the thermal wind sensor have been realized in standard CMOS process. The feasibility of the second-order thermal ?? modulator has been verified by the first proof-of-concept prototype. Measurements in wind tunnel show that the wind speed and direction accuracy are ±4% and ±2? respectively. Among the existing CMOS thermal wind sensors, the first-prototype achieves the lowest power consumption, which is 50mW in total. In the second prototype, system-level chopping is applied to reduce the offset of the interface circuit, to further reduce the power consumption to 25mW. Since the phase delay of the thermal filter is a monotonic function of the temperature, by measuring the peak frequency of the second-order thermal ?? modulator’s output square wave, temperature sensing can be achieved with the same thermal wind sensor chip. Measurements in the oven show that the resolution of the temperature sensing is about ±1?C with the range of [-40?C 50?C].