A polarization-agility antenna using liquid metal (eutectic gallium indium) with high radiation efficiency is proposed in this letter. This design approach offers possibilities to use the minimum amount of liquid metal as well as elastomer, so that a higher-radiation-efficiency liquid metal antenna can be realized. The proposed design is based on an aperture-coupled patch antenna composed of both liquid metal alloy and copper tape. The polarization of this antenna is controlled by the position of the pressure-driven liquid metal encased in four triangle cavities constructed by both the elastic dielectric material (i.e., Ecoflex) and polyethylene terephthalate film. A prototype is demonstrated with a center frequency of 2.45 GHz. Three kinds of polarization including left-hand circular polarization, right-hand circular polarization, and linear polarization are successfully demonstrated. A radiation efficiency higher than 90% is observed in the measurement.