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We designed and fabricated a novel microscale sensor for monitoring the motion of the gas-liquid interface in microchannels, based on the 3omega thermal-analysis method. As the sensor employs an AC hot-film technique to probe the movement of phase interface, it is more sensitive and noise-resistant than the already developed DC technique. The sensor is composed of a thin-film heater integrated into a PDMS microchannel fabricated on a glass substrate. The performance of the sensor is characterized by examining water injection and liquid evaporation processes. The results show good agreement with those by optical inspection. It is also demonstrated that the sensor can effectively monitor the long-term evaporation process of water in a microchannel. The result demonstrates strong potential of the proposed sensor as an integrated real-time probe to monitor a variety of microfluidic phenomena.