This paper reports an on-chip acoustic sensor comprising a piston-mode film bulk acoustic resonator and a monolithically integrated nanochannel. The resonator with the channel exhibits a resonance frequency (f) of 2.5 GHz and a quality (Q) factor of 436 in deionized water. The f �� Q product is as high as 1.1 �� 1012, which is the highest among all the acoustic wave sensors in the liquid phase. The sensor consumes 2 pl liquid volume and thus greatly saves the precious assays in biomedical testing. The Q factor is investigated, and real-time viscosity tests of glucose solution are demonstrated. The highly miniaturized and integrated sensor is capable to be arrayed with readout-circuitry, which opens an avenue for portable applications and lab-on-chip systems.