A planar U-shaped resonator for measuring relative permittivity of the liquid mediums is developed. To improve the sensitivity of the proposed sensor, gap sections, coupled lines and central ring resonators (RRs) are used to enhance electric fields interact with liquid under test (LUT). Further to increase the resonance sharpness, a tapered double-sided spiral split RR is presented. As the permittivity of the LUT is increased from 26 to 79, the measured resonance frequency of the described sensor is varied in the 166.4-287.5 MHz range. The developed sensor exhibits an improvement in sensitivity of about 45% compared with the other work.