In this paper, we developed a compact microdroplet monitoring sensor. This sensor is capacitive sensor combined a microfluidic circuit and a crystal oscillator. We succeeded in the generation and monitoring of microdroplets with this sensor. We optimized the sensing capacitor to improve the sensitivity of the sensor. With optimized the sensing capacitor, the frequency response can be increased. We measured NaCl aqueous solution concentration with the optimized sensing capacitor. From the measurement result, the high dynamic range measurement can be made by using two frequencies.