This paper reports the development of monolithic methanol concentration sensor for direct methanol fuel cell (DMFC). An interdigital capacitor and a resonator are fabricated on a single quartz crystal substrate. Methanol concentration can be measured by the frequency change of the resonator because the change corresponds to the dielectric constant change of the capacitor connected in series to the resonator. The smallest diameter of resonator and size of sensor that achieve a resolution of 0.1 wt% were 1.5 mm and 2.6×10.9×0.1 mm, respectively. Because the structure and the manufacturing process are simple, we expect it will be used for various kinds of liquid sensor. © 2012 IEEE.