<p>Micro thermoelectric device (μ-TED) emerges with great attention in energy generation, thermal management, and heat sensing applications. However, the large sensitive area is necessary to accommodate enough thermoelectric couples (TCs) for a high thermoelectric performance. This limits the potential in micro energy harvesting and ultra-sensitive sensing applications. Here, we adopted an optimized MEMS-based process to fabricate the ultra-sensitive micro-thermoelectric device (μ-TED). With the help of MEMS-compatible electrochemical deposition, the small size (25 μm), high aspect ratio (1:1.25), and alternating distributed P/N structures are achieved. As a result, the μ-TED realizes an ultra-high integration density of 19,900 thermoelectric couples per cm². Moreover, it shows a great thermoelectric sensitivity of 212 mV/(K·cm²) and a competitive power factor of 0.51 μW/(K²·cm²), which means the μ-TED is competent for miniaturized applications. Additionally, the μ-TED shows an ultra-low detection limit of 5 mm/s and a short response time of 100 ms, revealing great potential in fast detections of the ultra-low airflow. Furthermore, the ultra-sensitive μ-TED is utilized as a flexible breath sensor, due to its compact size. The breath signal of different motion states is successfully detected. These results confirm that the ultra-sensitive μ-TED holds outstanding potential for ultra-sensitive airflow sensing and energy harvesting devices.</p><p></p>