Abstract: A sensitive sensor for measurement of dielectric property small changes is proposed. Interference is used to cancel parasitic background signals to improve sensitivity. The device consists of two Wilkinson power dividers, a 180 degree reversephase and an inphase CPW (coplanar waveguide)slotline backtoback balun. The dielectric property small changes between deionize water and alcoholwater mixtures with different concentrations in terms of molar fractions are measured with clear sensing and identification. Experimental results show that the smallest absolute and relative permittivity changes are 1.68 and 2.07% respectively across the sensitive sensor, compared with microwave soil moisture sensors the proposed device is more sensitive. 