This paper introduces a kind of high-frequency liquid, transformer oil, which has low permittivity and dielectric loss. By employing this low-loss liquid material into structure of patch antenna, the property of frequency reconfigurability with high radiation efficiency is achieved. A two-layer substrate is here presented, where the upper one is made of homogeneous PTFE substrate for printing metal and the lower layer consists of a transparent container, including air and transformer oil layers. By varying the height of transformer oil layer, effective permittivity of the entire substrate underneath the patch can be tuned to realize frequency reconfiguration. Both linearly and circularly polarized (LP and CP) patch antenna prototypes are designed to verify the proposed methodology. The linearly polarized one is finally selected to be fabricated and tested. Measured results are found in reasonably good agreement with the simulated ones.