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This paper describes a novel design of a film bulk acoustic resonator (FBAR) without significant reduction of Q factor in liquid environments. The new design employs a vacuum gap to prevent acoustic energy loss in liquid. The Q factor was measured to be about 153 in the air and 140 in the water. We found that the Q factor is reduced only 9% in water, while the Q factor of a normal FBAR in the water is drastically decreased down to about 20 (more than 85% reduction). The device exhibited excellent liquid mass sensing capability due to the Q factor in liquid kept as high as in the air.